[1.8.x] Removed unnecessary F() from Func expressions docs

Backport of b792c4804b from master

docs/ref/models/expressions.txt

@@ -217,7 +217,7 @@ or they can be used to build a library of database functions::
     class Lower(Func):
         function = 'LOWER'
 
-    queryset.annotate(field_lower=Lower(F('field')))
+    queryset.annotate(field_lower=Lower('field'))
 
 But both cases will result in a queryset where each model is annotated with an
 extra attribute ``field_lower`` produced, roughly, from the following SQL::