[webidl] Grammar issues (#29)

I recognized various grammar issues:

1. The production Readonly is used several time but only ReadOnly exists
2. The production ReadonlyMemberRest uses Maplike and Setlike, but only MaplikeRest and SetlikeRest exist
3. The production BufferRelatedType contains the alternative "Int161Array" that should be "Int16Array"

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Reply to this email directly or view it on GitHub:
https://github.com/heycam/webidl/issues/29