Docs: noteable_type: 'Merge request' should be 'MergeRequest'

What does this MR do and why?

docs: noteable_type: 'Merge request' should be 'MergeRequest'

reference: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/note.rb#L359

related: https://github.com/jdalrymple/gitbeaker/pull/3491#discussion_r1432067089

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