Fix return type of listing methods in subreddit classes

[Lordshinjo]

This is an asyncpraw only change, fixing the return type of functions returning a ListingGenerator from Iterator to AsyncIterator in subreddit.py.

Typing was previously added as-is in #76 without adjustment for the different implementation of ListingGenerator between praw and asyncpraw.

[LilSpazJoekp]

Thanks for catching this!

Would you mind adding yourself to the Documentation Contributors section in the AUTHORS.rst file?

[Lordshinjo]

Would you mind adding yourself to the Documentation Contributors section in the AUTHORS.rst file?

Sure!

[LilSpazJoekp]

Thanks for catching that! 🎇