minor #13918 Fix Typo (menachemkorf)

This PR was submitted for the master branch but it was merged into the 4.4 branch instead.

Discussion
----------

Fix Typo

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/releases for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
-------

b18101b Update associations.rst

Author: javiereguiluz

doctrine/associations.rst

@@ -315,7 +315,7 @@ Saving Related Entities
 Now you can see this new code in action! Imagine you're inside a controller::
 
     // src/Controller/ProductController.php
-    namespace App/Controller;
+    namespace App\Controller;
 
     // ...
     use App\Entity\Category;