Remove mentions of nonexistent command brew diy

[thezeroalpha]

The output of brew doctor can mention the command brew diy, which was removed in this PR. It's better to remove this output to avoid confusing users. I replaced the text "...deleted or installed with `brew diy`" with "...deleted or installed manually", since manual installation of a program is usually what will lead to this message, but if you think something else would be better, let me know.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
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• Have you written new tests for your changes? Here's an example.
  • no, as far as I'm aware this change doesn't need a test
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• Have you successfully run brew tests with your changes locally?

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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, @thezeroalpha!

[thezeroalpha]

Thanks for the encouragement @MikeMcQuaid! Always happy to contribute to free software 😄