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

src/regexp.js

@@ -394,7 +394,7 @@ function SetUpRegExp() {
   // The length of compile is 1 in SpiderMonkey.
   %FunctionSetLength($RegExp.prototype.compile, 1);
 
-  // The properties input, $input, and $_ are aliases for each other.  When this
+  // The properties `input` and `$_` are aliases for each other.  When this
   // value is set the value it is set to is coerced to a string.
   // Getter and setter for the input.
   var RegExpGetInput = function() {