Fix minor wording issue.

sys.maxunicode is not called and thus does not return anything; it *is*
something.  (I checked the doc quickly to see if it tells that
expression return things but found nothing.)

I also removed markup that would just generate a useless link to the
enclosing section.

Doc/library/sys.rst

@@ -629,7 +629,7 @@ always available.
    i.e. ``1114111`` (``0x10FFFF`` in hexadecimal).
 
    .. versionchanged:: 3.3
-      Before :pep:`393`, :data:`sys.maxunicode` used to return either ``0xFFFF``
+      Before :pep:`393`, ``sys.maxunicode`` used to be either ``0xFFFF``
       or ``0x10FFFF``, depending on the configuration option that specified
       whether Unicode characters were stored as UCS-2 or UCS-4.