fix code tag on f_query

docs/f_query.rst

@@ -2,7 +2,7 @@ How to filter a queryset with criteria based on comparing their field values
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 Django ORM makes it easy to filter based on fixed values.
-To get all :code:`User` objects with :code:`first_name` starting with :code`'R'`,
+To get all :code:`User` objects with :code:`first_name` starting with :code:`'R'`,
 you can do :code:`User.objects.filter(name_startswith='R')`.
 
 What if you want to compare the first_name and last name?