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.

PR-URL: https://github.com/nodejs/node/pull/13134


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <mhdawson@ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>