[docs] typing.FrozenSet missing in documentation. (issue 28773)

[levkivskyi at gmail.com]

http://bugs.python.org/review/28773/diff/19258/Doc/library/typing.rst
File Doc/library/typing.rst (right):

http://bugs.python.org/review/28773/diff/19258/Doc/library/typing.rst#newcode587
Doc/library/typing.rst:587: .. class:: FrozenSet(frozenset,
AbstractSet[T])
FrozenSet is covariant in its type variable. PEP 8 recommends using
suffix ``_co`` for such type variables
https://www.python.org/dev/peps/pep-0008/#type-variable-names.

Therefore, I would write ``AbstractSet[T_co]`` instead of
``AbstractSet[T]``

http://bugs.python.org/review/28773/