Fix index example for TaggedJSONSerializer.register()

Made the example use a keyword, rather than a positional argument, as
the intent here is to show how to insert a tag with higher precedence.
As a positional argument, this would have been passed to ``force``
instead of ``index``.

flask/json/tag.py

@@ -36,7 +36,7 @@ processes dicts first, so insert the new tag at the front of the order since
         def to_python(self, value):
             return OrderedDict(value)
 
-    app.session_interface.serializer.register(TagOrderedDict, 0)
+    app.session_interface.serializer.register(TagOrderedDict, index=0)
 
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