Rename 'Build Hook' to 'Job Hook' in webhooks

Missing from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8787. Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/27805.

Implemented in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10586

We still refer to builds in https://docs.gitlab.com/ce/user/project/integrations/webhooks.html#build-events.