Add missing period at end of error message.

Author: jhgg

graphql/core/validation/rules.py

@@ -989,4 +989,4 @@ def enter_ObjectField(self, node, *args):
 
     @staticmethod
     def duplicate_input_field_message(field_name):
-        return 'There can only be one input field named "{}"'.format(field_name)
+        return 'There can only be one input field named "{}".'.format(field_name)