docs: update existing PR comments example

[tobiasbueschel]

Thanks for all your efforts with this github action 👏

This PR improves the example in the documentation by ensuring that the PR comment made is updated instead of creating a new comment. I realize that in some situations a new comment is desirable, however, this PR assumes that for most use cases updating an existing comment for a PR is desirable.

[hashicorp-cla]

All committers have signed the CLA.

[jpogran]

Thank you @tobiasbueschel for the contribution! I do think this is a valuable example to add, but also think it is valuable to have the prior example in addition to this one.

Can you leave the current example as is, and make your example an entirely new one underneath this? This way we take the explanation you put in the PR description inside the README and users can see the different ways you can configure this action.

[tobiasbueschel]

@jpogran absolutely, I've just updated the PR to reflect this :) Do let me know if there's anything else I should change. Thanks

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