docs: Don't reference the ZLib license in license-rules.rst

We never had a file called LICENSES/other/ZLib in the tree, so don't
reference it.  Instead mention the GPL v1 as an (bad) example.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>