docs: fix changelog header to consistent size (#147)

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

Author: dandhlee

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Changelog
 
-### [1.2.1](https://github.com/googleapis/python-api-gateway/compare/v1.2.0...v1.2.1) (2022-03-05)
+## [1.2.1](https://github.com/googleapis/python-api-gateway/compare/v1.2.0...v1.2.1) (2022-03-05)
 
 
 ### Bug Fixes
@@ -25,7 +25,7 @@
 
 * add autogenerated code snippets ([033bbc4](https://github.com/googleapis/python-api-gateway/commit/033bbc4c27b13e369df1b4527558f0a4de54fe80))
 
-### [1.1.1](https://www.github.com/googleapis/python-api-gateway/compare/v1.1.0...v1.1.1) (2021-11-01)
+## [1.1.1](https://www.github.com/googleapis/python-api-gateway/compare/v1.1.0...v1.1.1) (2021-11-01)
 
 
 ### Bug Fixes
@@ -51,14 +51,14 @@
 
 * improper types in pagers generation ([fc86cd1](https://www.github.com/googleapis/python-api-gateway/commit/fc86cd1c93a932c3c205524040d8547f70dbfb1c))
 
-### [1.0.4](https://www.github.com/googleapis/python-api-gateway/compare/v1.0.3...v1.0.4) (2021-09-24)
+## [1.0.4](https://www.github.com/googleapis/python-api-gateway/compare/v1.0.3...v1.0.4) (2021-09-24)
 
 
 ### Bug Fixes
 
 * add 'dict' annotation type to 'request' ([39ee308](https://www.github.com/googleapis/python-api-gateway/commit/39ee308769c9d999acca90c5ac56e4cd5806c822))
 
-### [1.0.3](https://www.github.com/googleapis/python-api-gateway/compare/v1.0.2...v1.0.3) (2021-07-27)
+## [1.0.3](https://www.github.com/googleapis/python-api-gateway/compare/v1.0.2...v1.0.3) (2021-07-27)
 
 
 ### Bug Fixes
@@ -75,22 +75,22 @@
 
 * release as 1.0.3 ([#53](https://www.github.com/googleapis/python-api-gateway/issues/53)) ([aa6e493](https://www.github.com/googleapis/python-api-gateway/commit/aa6e49383d2df7b88282ee9fe032e802758a564f))
 
-### [1.0.2](https://www.github.com/googleapis/python-api-gateway/compare/v1.0.1...v1.0.2) (2021-07-21)
+## [1.0.2](https://www.github.com/googleapis/python-api-gateway/compare/v1.0.1...v1.0.2) (2021-07-21)
 
 
 ### Bug Fixes
 
 * **deps:** pin 'google-{api,cloud}-core', 'google-auth' to allow 2.x versions ([#47](https://www.github.com/googleapis/python-api-gateway/issues/47)) ([17aed88](https://www.github.com/googleapis/python-api-gateway/commit/17aed8856fd9143e141ffc5c6ff51c0fc8937743))
 
-### [1.0.1](https://www.github.com/googleapis/python-api-gateway/compare/v1.0.0...v1.0.1) (2021-07-12)
+## [1.0.1](https://www.github.com/googleapis/python-api-gateway/compare/v1.0.0...v1.0.1) (2021-07-12)
 
 
 ### Bug Fixes
 
 * disable always_use_jwt_access ([b2869e6](https://www.github.com/googleapis/python-api-gateway/commit/b2869e6fc540d65af1bdcf4329f8c888ab61de9a))
 * disable always_use_jwt_access ([#41](https://www.github.com/googleapis/python-api-gateway/issues/41)) ([b2869e6](https://www.github.com/googleapis/python-api-gateway/commit/b2869e6fc540d65af1bdcf4329f8c888ab61de9a))
 
-### [1.0.1](https://www.github.com/googleapis/python-api-gateway/compare/v1.0.0...v1.0.1) (2021-06-30)
+## [1.0.1](https://www.github.com/googleapis/python-api-gateway/compare/v1.0.0...v1.0.1) (2021-06-30)
 
 
 ### Bug Fixes