Remove mention of `RegExp.$input`

src/regexp.js

Index: src/regexp.js
diff --git a/src/regexp.js b/src/regexp.js
index 1a2a4f0eb711a71d27ada0c08975dee2e6f886dc..d8e3ea49bec7e236a0d7dc1dcf4a624b460679eb 100644
--- a/src/regexp.js
+++ b/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() {