Correct two nomenclature errors
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

resin-io/herostratus,resin-io/resin-etcher,resin-io/resin-etcher,resin-io/herostratus,resin-io/etcher,resin-io/etcher,resin-io/etcher,resin-io/etcher,resin-io/etcher