.APP vs. .DMG file in Mac OS quickstart, per #61

[gregkrsak]

Mac OS quick start instructions say that Nestopia is available via a link to a .DMG file, but it is really an .APP.

[gregkrsak]

See Issue #61

[gregkrsak]

Fix submitted in 695441f

Requesting someone with Mac OS verify.

[JamesSkemp]

I can verify that the link works (macOS Catalina, 10.15.6). Still no movement from the character, however. (Keys are mapped to the right and left arrow by default.)