maint: small updates to Yubikey guide
There are 2 small changes being made here:
- Instead of using
umount
to unmount the encrypted GPG master volume on MacOS,hdiutil detach
should be used instead. Explanation here: https://stackoverflow.com/a/4047528 - Moved the Cleanup step higher, to just after the Testing step, as this seems like a more logical flow. I almost missed the cleanup step when I was going through the guide.
- Also make the formatting nicer on that step.