minor symfony#3770 Update factories.rst (AlaaAttya)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes symfony#3770).

Discussion
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Update factories.rst

your factory method should be static to work!

Commits
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97ed5ce Update factories.rst
aefc36c Update factories.rst
a9cfdb2 Update factories.rst

Author: weaverryan

components/dependency_injection/factories.rst

@@ -79,8 +79,8 @@ class:
 
 When you specify the class to use for the factory (via ``factory_class``)
 the method will be called statically. If the factory itself should be instantiated
-and the resulting object's method called (as in this example), configure the
-factory itself as a service:
+and the resulting object's method called, configure the factory itself as a service.
+In this case, the method (e.g. get) should be changed to be non-static:
 
 .. configuration-block::