Fix minor doc bug on Repository.iter_issues

I saw the `sort` parameter in Github's documentation, but was wondering where it was in github3.py.  It turns out it's there, both supported and documented, but the doc line was mistakenly merged into the previous line, which makes it easy to miss.

Now it should be more hunky-dory.
Merge pull request #214 from xiongchiamiov/develop

Fix minor doc bug on Repository.iter_issues