Fix finding codeownersDirectory on windows with parent folder

[noahm]

Hey, long time! This library has continued to be super useful at Twitch over the years! We happened to notice today that the regex I added years ago wasn't correctly matching when run on a windows environment (uses a backslash) and resulted in the wrong codeownersDirectory being set when used in a repo that put the CODEOWNERS file under one of the parent folders.

This may also fix #16 depending on what the reporter's original issue was, but without more context that's impossible to say.