tools: replace loop with padStart

PR-URL: nodejs/node#16220
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

Author: tniessen

tools/license2rtf.js

@@ -274,10 +274,7 @@ function RtfGenerator() {
   };
 
   function toHex(number, length) {
-    var hex = (~~number).toString(16);
-    while (hex.length < length)
-      hex = `0${hex}`;
-    return hex;
+    return (~~number).toString(16).padStart(length, '0');
   }
 
   function rtfEscape(string) {