rust: block: fix `srctree/` links

commit 208d7f78 upstream.

This `srctree/` link pointed to a file with an underscore, but the header
used a dash instead.

Thus fix it.

This cleans a future warning that will check our `srctree/` links.

Cc: stable@vger.kernel.org
Fixes: 3253aba3 ("rust: block: introduce `kernel::block::mq` module")
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>