chore(repo): typo in mergify queue message (#3876)

idk why I thought the `@` was part of the interpolation syntax

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.mergify.yml

@@ -16,7 +16,7 @@ pull_request_rules:
     actions:
       comment:
         message: |
-          Thanks for contributing, @{{author}}! This PR will now be added to the [merge queue](https://mergify.com/merge-queue), or immediately merged if @{{head}} is up-to-date with @{{base}} and the queue is empty.
+          Thanks for contributing, @{{author}}! This PR will now be added to the [merge queue](https://mergify.com/merge-queue), or immediately merged if `{{head}}` is up-to-date with `{{base}}` and the queue is empty.
       queue:
         name: default
         method: squash