Update mpv.rb

[robertleeblairjr]

Unsure if this is correct for assisting to update FFmpeg 4.2 PR. Bumping revision for mpv.

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

---

[SMillerDev]

Revision bumps have to be done in the same PR.

[retokromer]

#42827