Fixed the API calls for watching and unwatching a repo

As per https://developer.github.com/v3/activity/watching/, it seems the "watched" part of the URL to handle watching and unwatching a repo should be "subscriptions" instead.  The updated URL (in this pull request) worked fine for me; the original one didn't.
It would be better to write it as ``@deprecated use unsubscribe() instead``, to match common deprecation messages