virtualbox: give full url for downloading the ext (#15869)

The user only has to agree on the terms and conditions before
downloading the file. We might as well give him access to the full URL
by default.

pkgs/applications/virtualization/virtualbox/default.nix

···
     sha256 = "11f40842a56ebb17da1bbc82a21543e66108a5330ebd54ded68038a990aa071b";
     message = ''
       In order to use the extension pack, you need to comply with the VirtualBox Personal Use
-      and Evaluation License (PUEL) by downloading the related binaries from:
+      and Evaluation License (PUEL) available at:
 
-      https://www.virtualbox.org/wiki/Downloads
+      https://www.virtualbox.org/wiki/VirtualBox_PUEL
 
-      Once you have downloaded the file, please use the following command and re-run the
-      installation:
+      Once you have read and if you agree with the license, please use the
+      following command and re-run the installation:
 
-      nix-prefetch-url file://${name}
+      nix-prefetch-url http://download.virtualbox.org/virtualbox/${version}/${name}
     '';
   };