doc: Update small error in LICENSE for npm

This corresponds to 6051a69b1a

PR-URL: https://github.com/nodejs/node/pull/4872
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rebecca Turner <me@re-becca.org>

LICENSE

@@ -680,7 +680,7 @@ The externally maintained libraries used by Node.js are:
     Copyright (c) their respective copyright owners
     Licensed on their respective license terms
 
-    The npm public registry at https://registry.npmjs.com
+    The npm public registry at https://registry.npmjs.org
     and the npm website at https://www.npmjs.com
     Operated by npm, Inc.
     Use governed by terms published on https://www.npmjs.com