Fix typo (GH-21820)

Author: chrisyeh96

Doc/library/stdtypes.rst

@@ -4205,7 +4205,7 @@ The constructors for both classes work the same:
 
 
    Note, the non-operator versions of :meth:`union`, :meth:`intersection`,
-   :meth:`difference`, and :meth:`symmetric_difference`, :meth:`issubset`, and
+   :meth:`difference`, :meth:`symmetric_difference`, :meth:`issubset`, and
    :meth:`issuperset` methods will accept any iterable as an argument.  In
    contrast, their operator based counterparts require their arguments to be
    sets.  This precludes error-prone constructions like ``set('abc') & 'cbs'``