Update basic factory example and docs

Author: rmk135

docs/images/providers/factory.png

@@ -1,7 +1,7 @@
 Factory providers
 -----------------
 
-``Factory`` provider creates new instance of specified class on every call.
+``di.Factory`` provider creates new instance of specified class on every call.
 
 Nothing could be better than brief example:
 

docs/providers/factory.rst

@@ -1,14 +1,14 @@
-"""`Factory` providers example."""
+"""`di.Factory` providers example."""
 
-from dependency_injector.providers import Factory
+import dependency_injector as di
 
 
 class User(object):
 
     """Example class User."""
 
 # Factory provider creates new instance of specified class on every call.
-users_factory = Factory(User)
+users_factory = di.Factory(User)
 
 # Creating several User objects:
 user1 = users_factory()