fix typo in CSS docs
fix typo in CSS docs

BS3 commit 9546fc9cf004d38f7aa4d2b64e8ab6bc65920718

todc/todc-bootstrap,todc/todc-bootstrap,ztx1491/todc-bootstrap,up2vs/todc-bootstrap,2947721120/googlebt4,todc/todc-bootstrap,2947721120/todc-bootstrap,aaroncnhk/todc-bootstrap,mozillo/todc-bootstrap