Corrected type of IsMature field on channe.update event

[zneix]

I received this json unmarshal error while trying to parse a notification received from Twitch on the EventSub's channel.update event:

2021/08/27 19:20:41 [EventSub] Failed to unmarshal notification event: json: cannot unmarshal bool into Go struct field EventSubChannelUpdateEvent.is_mature of type string, data: {"broadcaster_user_id":"97916945","broadcaster_user_login":"heryin","broadcaster_user_name":"Heryin","title":"Testin again","language":"en","category_id":"509110","category_name":"Risk of Rain 2","is_mature":false}

Seems like returned is_mature is a boolean, according to the EventSub documentation.

[coveralls]

Pull Request Test Coverage Report for Build 1175148509

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.639%

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Totals
Change from base Build 1072491672:	0.0%
Covered Lines:	1064
Relevant Lines:	1101

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💛 - Coveralls