Update language around updating branch with latest master (#1885)

Resolves #1884 

In a recent community commit merge to Pro, we had [a
question](tiny-pilot/tinypilot-pro#1509 (comment))
around which style of merge to use (regular or rebase) when updating the
branch with Pro's `master`.

This change updates the body text of community-changes PRs to suggest
using "Update with merge commit" when updating the branch.

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Author: db39

.github/workflows/merge-pro.yml

@@ -45,6 +45,7 @@ jobs:
             @${{ github.actor }}, please merge [your changes from the Community repository](https://github.com/tiny-pilot/tinypilot/commit/${{ github.sha }}):
 
             1. Update this branch with latest `master`
+               - Generally, you can use GitHub’s “Update with merge commit”.
                - If there are merge conflicts, it’s best to resolve them locally.
             1. Check whether you need to make any Pro-specific adjustments to the code
             1. Review and approve your changes