Fix PR comment deserialization

[Mark-Simulacrum]

Broke in #1646.

request failed: WebhookError(IssueCommentEvent failed to deserialize

Caused by:
    0: at Path { segments: [Map { key: "issue" }, Map { key: "pull_request" }] }
    1: invalid type: map, expected a boolean at line 1 column 6086)

[ehuss]

Apologies! In hindsight that was an obvious mistake and I should have reviewed the API more closely. Thanks for the fix!