doc: change STYLE-GUIDE to STYLE_GUIDE

PR-URL: #11460
Fixes: #11456
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Claudio Rodriguez <[email protected]>
Reviewed-By: Gibson Fahnestock <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Joyee Cheung <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>

Author: Dean-Coakley