minor #11689 Fix markup (stof)

This PR was merged into the 3.4 branch.

Discussion
----------

Fix markup

The extra indentation made it a blockquote:

![symfony_doc_markup](https://user-images.githubusercontent.com/439401/59041189-6eed2880-8878-11e9-8345-d128098c9936.png)

Commits
-------

7cc97d9 Fix markup

Author: wouterj

contributing/code/pull_requests.rst

@@ -129,7 +129,7 @@ work:
   </contributing/code/maintenance>` (you may have to choose a higher branch if
   the feature you are fixing was introduced in a later version);
 
- * ``master``, if you are adding a new feature.
+* ``master``, if you are adding a new feature.
 
 .. note::