Use correct example of a Dashicons helper class in add_menu_page() and register_post_type() docs.

props Rarst.
fixes #26617 for trunk.
Built from https://develop.svn.wordpress.org/trunk@26884


git-svn-id: https://core.svn.wordpress.org/trunk@26767 1a063a9b-81f0-0310-95a4-ce76da25c4cd