Specify that Session.headers needs to be set to a OrderedDict in Head…

…er Ordering docs (#6475)

docs/user/advanced.rst

@@ -1081,7 +1081,7 @@ Header Ordering
 
 In unusual circumstances you may want to provide headers in an ordered manner. If you pass an ``OrderedDict`` to the ``headers`` keyword argument, that will provide the headers with an ordering. *However*, the ordering of the default headers used by Requests will be preferred, which means that if you override default headers in the ``headers`` keyword argument, they may appear out of order compared to other headers in that keyword argument.
 
-If this is problematic, users should consider setting the default headers on a :class:`Session <requests.Session>` object, by setting :attr:`Session <requests.Session.headers>` to a custom ``OrderedDict``. That ordering will always be preferred.
+If this is problematic, users should consider setting the default headers on a :class:`Session <requests.Session>` object, by setting :attr:`Session.headers <requests.Session.headers>` to a custom ``OrderedDict``. That ordering will always be preferred.
 
 .. _timeouts: