Update doctrine.rst

Reworded, since "this" was ambiguous: Could refer to the setter as well (i.e. the setter gets created).

Author: ThomasLandauer

doctrine.rst

@@ -316,7 +316,7 @@ then go to the Code -> Generate menu and select "Getters and Setters"::
 
 .. tip::
 
-    Typically you won't need a ``setId()`` method: Doctrine will set this for you
+    Typically you won't need a ``setId()`` method: Doctrine will set the ID
     automatically.
 
 Persisting Objects to the Database