doc: update URL for sparse's tarballs

The URL given to get sparse's tarballs is sparse.wiki.kernel.org
but the wiki will is deprecated in favor of sparse.docs.kernel.org.

So, update this URL to the one where the tarballs can directly
be found.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Link: https://lore.kernel.org/r/20200621153330.54480-1-luc.vanoostenryck@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>