Remove mention of `RegExp.$input`

R=yangguo@chromium.org, arv@chromium.org
BUG=v8:3486
LOG=N

Review URL: https://codereview.chromium.org/503703002

Patch from Mathias Bynens <mathias@qiwi.be>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23337 ce2b1a6d-e550-0410-aec6-3dcde31c8c00