doc: change STYLE-GUIDE to STYLE_GUIDE

PR-URL: #11460
Fixes: #11456
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

Author: Dean-Coakley