Rename UNICODE_MODE_MAC in docs to UNICODE_MODE_MACOS

[JasonHiew]

By following the documentation's example, the firmware doesn't compile and returns this error:

error: 'UNICODE_MODE_MAC' undeclared here (not in a function); did you mean 'UNICODE_MODE_EMACS'?

The docs should be updated from

#define UNICODE_SELECTED_MODES UNICODE_MODE_MAC, UNICODE_MODE_WINCOMPOSE

to

#define UNICODE_SELECTED_MODES UNICODE_MODE_MACOS, UNICODE_MODE_WINCOMPOSE

and

Mode Name: UNICODE_MODE_MAC to Mode Name: UNICODE_MODE_MACOS