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: #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>

doc/STYLE_GUIDE.md

@@ -39,7 +39,7 @@
 * When documenting APIs, note the version the API was introduced in at
   the end of the section. If an API has been deprecated, also note the first
   version that the API appeared deprecated in.
-* When using dashes, use emdashes ("—", Ctrl+Alt+"-" on macOS) surrounded by
+* When using dashes, use emdashes ("—", Option+Shift+"-" on macOS) surrounded by
   spaces, per the New York Times usage.
 * Including assets:
   * If you wish to add an illustration or full program, add it to the