Make 'UPSTREAM_REF' a variable in the example

What does this MR do and why?

This MR updates the pass artifacts from a Merge Request pipeline example to clarify that $UPSTREAM_REF should be a variable (rather than the string UPSTREAM_REF).

GitLab team members with access to Slack can view the origin of this example and observe that it was intended to be $UPSTREAM_REF there.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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