Fix reference to default IRC hook events.

IRC hooks only listen to [`push` and `pull_request` events](https://github.com/github/github-services/blob/master/lib/services/irc.rb#L7) by default. This was causing confusion for users who were expecting to receive `issues` events as well.

There may be a better hook to use as an example since the list of supported events for the IRC hooks varies so much from the default set.