faq: add missing brew install flag

[gromgit]

Otherwise brew install keeps installing from bottle and ignoring users' edits.

Addresses https://github.com/orgs/Homebrew/discussions/2394#discussioncomment-10393518

• 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?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

[MikeMcQuaid]

Good catch, thanks @gromgit!