[brew audit] fix "Incorrect file permissions" message

[jesboat]

When the file isn't world-readable, brew audit prints a failure message including a suggestion to chmod +r the file. Unfortunately, this isn't quite right: with both macOS and coreutils, leaving out the "who" in a chmod only affects bits which would be set in the umask. So, if the umask doesn't allow world-readable (which might be why the file wasn't world-readable in the first place), the suggested chmod command does nothing.

Change to print chmod a+r instead; that does have the intended effect.

No other chmod suggestions in this file have the same problem.

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[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @jesboat!