Update wording and punctuation in _auto_devops_callout.html.haml

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What does this MR do and why?

Remove two extraneous words in the banner text to keep the language parallel with the second imperative sentence. (Plus, this just sounds better.) Also, finish the second sentence with a period.

Before: It will automatically build, test, and deploy...

After: Automatically build, test, and deploy...

(This banner appears on repositories that don't have Auto DevOps enabled.)

MR acceptance checklist

No code changes, only a UI text string. No changelog trailers required, as far as I can tell.

Screenshots or screen recordings

Not required, I assume.

How to set up and validate locally

Text-only change to the banner. No code changes.