Fixes method in docs for MergeRequests#cancel_merge_when_pipeline_succeeds API call

What does this MR do?

The actual API uses the POST method instead of PUT: https://gitlab.com/gitlab-org/gitlab/blob/bf24aef3d8a3b422c56f4d6f29914708b7ceb38d/lib/api/merge_requests.rb#L447.

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• [-] If applicable, update the permissions table.
• [-] Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

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3. Maintainer

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