docs: corrections (#9189) · ZcashFoundation/zebra@bb7ef30

"pull_request_target.opened" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

"pull_request_target.reopened" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

"pull_request_target.synchronize" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

"pull_request_target.edited" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

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