use correct+newer name for retina mixin in docs; fixes #12450
use correct+newer name for retina mixin in docs; fixes #12450

zackn9ne/Franco,GiorgioPingendo/bootstrap-1,zackn9ne/Franco,zackn9ne/Franco,GiorgioPingendo/bootstrap-1,GiorgioPingendo/bootstrap-1,zackn9ne/Franco,GiorgioPingendo/bootstrap-1