Twenty Fifteen: Reflect the change from "no" to "share" icon in [30810] in readme.txt.

props iamtakashi.
fixes #30148.
Built from https://develop.svn.wordpress.org/trunk@30828


git-svn-id: http://core.svn.wordpress.org/trunk@30818 1a063a9b-81f0-0310-95a4-ce76da25c4cd