minor symfony#8238 Typo fixed in a doctrine docs (pavemaksim)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes symfony#8238).

Discussion
----------

Typo fixed in a doctrine docs

Commits
-------

3100ab7 Typo fixed in a doctrine docs

Author: xabbuh

doctrine.rst

@@ -632,7 +632,7 @@ Updating an Object
 Once you've fetched an object from Doctrine, updating it is easy. Suppose
 you have a route that maps a product id to an update action in a controller::
 
-    use AppBundle\Entity\Post;
+    use AppBundle\Entity\Product;
     // ...
 
     public function updateAction($productId)