docs: dev-tools/lkmm: Fix typo of missing file extension

docs: dev-tools/lkmm: Fix typo of missing file extension

Commit 1e9ddbb2cd34 ("docs: Pull LKMM documentation into dev-tools book")
failed to add a file extension in lkmm/docs/herd-representation.rst for
referencing its plane-text counterpart.

Fix it.

Fixes: 1e9ddbb2cd34 ("docs: Pull LKMM documentation into dev-tools book")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202509192138.fx3H6NzG-lkp@intel.com/
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <c3b9de17-7cd8-4968-9872-cbe2607a7143@gmail.com>