doc: fix incorrect keyboard shortcut
This commit fixes an incorrect keyboard shortcut in doc/STYLE_GUIDE.md
: entering em-dashes is done via Alt+Shift+"-" on macOS, not via Ctrl+Alt+"-". Besides that, Option is more canonical name of Alt on Macs.
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc