[Contributing] Replace mentions of 2.2 for 2.3

As per WouterJ request: symfony#3421 (comment)

Author: lemoinem

contributing/code/patches.rst

@@ -107,13 +107,13 @@ Choose the right Branch
 Before working on a patch, you must determine on which branch you need to
 work. The branch should be based on the ``master`` branch if you want to add a
 new feature. But if you want to fix a bug, use the oldest but still maintained
-version of Symfony where the bug happens (like ``2.2``).
+version of Symfony where the bug happens (like ``2.3``).
 
 .. note::
 
  All bug fixes merged into maintenance branches are also merged into more
  recent branches on a regular basis. For instance, if you submit a patch
- for the ``2.2`` branch, the patch will also be applied by the core team on
+ for the ``2.3`` branch, the patch will also be applied by the core team on
  the ``master`` branch.
 
 Create a Topic Branch
@@ -126,18 +126,18 @@ topic branch:
 
  $ git checkout -b BRANCH_NAME master
 
-Or, if you want to provide a bugfix for the ``2.2`` branch, first track the remote
-``2.2`` branch locally:
+Or, if you want to provide a bugfix for the ``2.3`` branch, first track the remote
+``2.3`` branch locally:
 
 .. code-block:: bash
 
- $ git checkout -t origin/2.2
+ $ git checkout -t origin/2.3
 
-Then create a new branch off the ``2.2`` branch to work on the bugfix:
+Then create a new branch off the ``2.3`` branch to work on the bugfix:
 
 .. code-block:: bash
 
- $ git checkout -b BRANCH_NAME 2.2
+ $ git checkout -b BRANCH_NAME 2.3
 
 .. tip::
 
@@ -228,7 +228,7 @@ while to finish your changes):
 
 .. tip::
 
- Replace ``master`` with the branch you selected previously (e.g. ``2.2``)
+ Replace ``master`` with the branch you selected previously (e.g. ``2.3``)
  if you are working on a bugfix
 
 When doing the ``rebase`` command, you might have to fix merge conflicts.
@@ -253,8 +253,8 @@ You can now make a pull request on the ``symfony/symfony`` Github repository.
 
 .. tip::
 
- Take care to point your pull request towards ``symfony:2.2`` if you want
- the core team to pull a bugfix based on the ``2.2`` branch.
+ Take care to point your pull request towards ``symfony:2.3`` if you want
+ the core team to pull a bugfix based on the ``2.3`` branch.
 
 To ease the core team work, always include the modified components in your
 pull request message, like in: