Indicate required content-type for webhook subscription verification

[Nicolas-Reyland]

Since the documentation doesn't tell it (I looked thoroughly, but I might be wrong), I think it's a good idea to tell the user which content-type is awaited when verifying the webhook subscription.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Problem/Feature

The problem I identified (maybe wrongly?) is that it is not specified in the official twitch api documentation which value the Content-Type header should have for the webhook subscription verification. Since I had to look into the source code of this project to get my answer (no hard feelings at all), I think it might be good to indicate to the users of the twitch-cli why their requests are invalid.

Description of Changes:

• Added the expected value in the two error messages that indicate that the Content-Type doesn't have the right value.

Checklist

• My code follows the Contribution Guide
• I have self-reviewed the changes being requested
• I have made comments on pieces of code that may be difficult to understand for other editors
• I have updated the documentation (if applicable)

[BarryCarlyon]

Seems good! If only i had write access!

Also a thing that might need to be changed/tweaked in the main Twitch Docs for EventSub