docs: fix changelog header to consistent size (#205)

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Author: dandhlee

CHANGELOG.md

@@ -7,7 +7,7 @@
 
 * update protos to include InvalidateApprovalRequest and GetAccessApprovalServiceAccount APIs ([#199](https://github.com/googleapis/python-access-approval/issues/199)) ([908cdc3](https://github.com/googleapis/python-access-approval/commit/908cdc31d1625a5bc66a3623ea11e76acddb4f62))
 
-### [1.5.1](https://github.com/googleapis/python-access-approval/compare/v1.5.0...v1.5.1) (2022-03-05)
+## [1.5.1](https://github.com/googleapis/python-access-approval/compare/v1.5.0...v1.5.1) (2022-03-05)
 
 
 ### Bug Fixes
@@ -33,7 +33,7 @@
 * add autogenerated code snippets ([8ca51b3](https://github.com/googleapis/python-access-approval/commit/8ca51b375650cf3d1374beee431a27eb6cb7e81d))
 * added resource annotations ([#161](https://github.com/googleapis/python-access-approval/issues/161)) ([de63e9e](https://github.com/googleapis/python-access-approval/commit/de63e9e0cdc66983096bb2714a9204b07e85811f))
 
-### [1.4.1](https://www.github.com/googleapis/python-access-approval/compare/v1.4.0...v1.4.1) (2021-11-01)
+## [1.4.1](https://www.github.com/googleapis/python-access-approval/compare/v1.4.0...v1.4.1) (2021-11-01)
 
 
 ### Bug Fixes
@@ -55,21 +55,21 @@
 * add context manager support in client ([#126](https://www.github.com/googleapis/python-access-approval/issues/126)) ([372628b](https://www.github.com/googleapis/python-access-approval/commit/372628b04734c8a15a0ed0ada374120a4ce024db))
 * add trove classifier for python 3.9 and python 3.10 ([#129](https://www.github.com/googleapis/python-access-approval/issues/129)) ([2bb4981](https://www.github.com/googleapis/python-access-approval/commit/2bb4981fc62bfbf918e0c215e97f1edb47dcc045))
 
-### [1.3.5](https://www.github.com/googleapis/python-access-approval/compare/v1.3.4...v1.3.5) (2021-09-30)
+## [1.3.5](https://www.github.com/googleapis/python-access-approval/compare/v1.3.4...v1.3.5) (2021-09-30)
 
 
 ### Bug Fixes
 
 * improper types in pagers generation ([a73b1e0](https://www.github.com/googleapis/python-access-approval/commit/a73b1e0bc466b6edc6355423180b4cf44d64e55a))
 
-### [1.3.4](https://www.github.com/googleapis/python-access-approval/compare/v1.3.3...v1.3.4) (2021-09-24)
+## [1.3.4](https://www.github.com/googleapis/python-access-approval/compare/v1.3.3...v1.3.4) (2021-09-24)
 
 
 ### Bug Fixes
 
 * add 'dict' annotation type to 'request' ([0585562](https://www.github.com/googleapis/python-access-approval/commit/0585562b62c7c7fe81eacc8124ab4f85484ee879))
 
-### [1.3.3](https://www.github.com/googleapis/python-access-approval/compare/v1.3.2...v1.3.3) (2021-07-27)
+## [1.3.3](https://www.github.com/googleapis/python-access-approval/compare/v1.3.2...v1.3.3) (2021-07-27)
 
 
 ### Bug Fixes
@@ -86,14 +86,14 @@
 
 * release as 1.3.3 ([#103](https://www.github.com/googleapis/python-access-approval/issues/103)) ([0b964c6](https://www.github.com/googleapis/python-access-approval/commit/0b964c60b8306d53e364a4431be55bebf1c48b48))
 
-### [1.3.2](https://www.github.com/googleapis/python-access-approval/compare/v1.3.1...v1.3.2) (2021-07-20)
+## [1.3.2](https://www.github.com/googleapis/python-access-approval/compare/v1.3.1...v1.3.2) (2021-07-20)
 
 
 ### Bug Fixes
 
 * **deps:** pin 'google-{api,cloud}-core', 'google-auth' to allow 2.x versions ([#97](https://www.github.com/googleapis/python-access-approval/issues/97)) ([9400ffe](https://www.github.com/googleapis/python-access-approval/commit/9400ffeffb89458f738961ef0ef4729141f37ee8))
 
-### [1.3.1](https://www.github.com/googleapis/python-access-approval/compare/v1.3.0...v1.3.1) (2021-06-30)
+## [1.3.1](https://www.github.com/googleapis/python-access-approval/compare/v1.3.0...v1.3.1) (2021-06-30)
 
 
 ### Bug Fixes
@@ -128,7 +128,7 @@
 * **deps:** add packaging requirement ([#78](https://www.github.com/googleapis/python-access-approval/issues/78)) ([fd1417b](https://www.github.com/googleapis/python-access-approval/commit/fd1417b61019150963c83b4594fd86e2fb304355))
 * use correct retry deadlines ([#57](https://www.github.com/googleapis/python-access-approval/issues/57)) ([8f6833a](https://www.github.com/googleapis/python-access-approval/commit/8f6833aa95e7610576fba4500857a4143de7d1d7))
 
-### [1.1.1](https://www.github.com/googleapis/python-access-approval/compare/v1.1.0...v1.1.1) (2021-02-12)
+## [1.1.1](https://www.github.com/googleapis/python-access-approval/compare/v1.1.0...v1.1.1) (2021-02-12)
 
 
 ### Bug Fixes