Fix to incorrect example in pipelines.md

What does this MR do?

Fixes error in documentation for Get the Latest Pipeline in the Pipelines API documentation changing the request type to a GET request which the API expects.

Related issues

None.

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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  • Style Guide.
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Reviewer's checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
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    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
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