minor #5011 tip for mapping definition (SrgSteak)

This PR was merged into the 2.3 branch.

Discussion
----------

tip for mapping definition

If you follow the link from the security documentation (http://symfony.com/doc/current/book/security.html#loading-users-from-the-database), it's easy to miss the necessary mapping definition for the repository.

Commits
-------

7ae3a28 improved link
38dcfc8 fixed intendation and shortened the link
4422c10 tip for mapping definition

cookbook/security/entity_provider.rst

@@ -583,6 +583,11 @@ The code below shows the implementation of the
         }
     }
 
+.. tip::
+
+    Don't forget to add the repository class to the 
+    :ref: `mapping definition of your entity <book-doctrine-custom-repository-classes>`.
+
 To finish the implementation, the configuration of the security layer must be
 changed to tell Symfony to use the new custom entity provider instead of the
 generic Doctrine entity provider. It's trivial to achieve by removing the