Remove "repository"

Brings `create` in line with `delete`. It seems notification of a repository being created doesn't make sense since you have to set up the webhook on an already created repository. I asked support and Jamie Murai wrote back to say

>Thanks for letting us know about this. That's a mistake in the documentation. At this time, you cannot get "create" events when a repo is created. This is because a webhook must be added to a repository, so the repository must already exist.
Merge pull request #518 from thomasballinger/patch-1

Remove "repository" from create webhook event description