Recommend app_ instead of app. as form type prefix

Author: javiereguiluz

book/forms.rst

@@ -1063,7 +1063,7 @@ that will house the logic for building the task form::
     identifiers must be unique in the application. Unless you want to override
     a built-in type, they should be different from the default Symfony types
     and from any type defined by a third-party bundle installed in you application.
-    Consider prefixing your types with ``app.`` to avoid identifier collisions.
+    Consider prefixing your types with ``app_`` to avoid identifier collisions.
 
 This new class contains all the directions needed to create the task form. It can
 be used to quickly build a form object in the controller::