message: Correct two nomenclature errors
  PC keyboards have "Alt" keys; Mac keyboards have "Opt" keys.
  Although it's possible to use a PC keyboard on a Mac, it's unusual.
  In any case, all of the macOS (not "Mac OS" for some years now) documentation refers to the "Opt" key.
  Change-type: patch
diff: - **Image flashed:**
  - **Do you see any meaningful error information in the DevTools?**
  -<!-- You can open DevTools by pressing `Ctrl+Shift+I` (`Ctrl+Alt+I` for Etcher before v1.3.x), or `Cmd+Alt+I` if you're on Mac OS. -->
  +<!-- You can open DevTools by pressing `Ctrl+Shift+I` (`Ctrl+Alt+I` for Etcher before v1.3.x), or `Cmd+Opt+I` if you're on macOS. -->
  