Fix grammar in changed? docs [ci skip]

See https://english.stackexchange.com/questions/23218/anyone-has-or-anyone-have-seen-them

activemodel/lib/active_model/dirty.rb

@@ -151,7 +151,7 @@ def changes_applied # :nodoc:
       @mutations_from_database = nil
     end
 
-    # Returns +true+ if any of the attributes have unsaved changes, +false+ otherwise.
+    # Returns +true+ if any of the attributes has unsaved changes, +false+ otherwise.
     #
     #   person.changed? # => false
     #   person.name = 'bob'