Fix discrepancy in merge request approval rules documentation

What does this MR do and why?

approvals_required and name are optional for PUT /projects/:id/merge_requests/:merge_request_iid/approval_rules/:approval_rule_id.

Related code: https://gitlab.com/gitlab-org/gitlab/blob/0af1a960d26c9b0f9d65df3da970a81855445cbb/ee/lib/api/merge_request_approval_rules.rb#L79-85

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