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

Commit 1e9ddbb ("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: 1e9ddbb ("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>

Author: akiyks

Documentation/dev-tools/lkmm/docs/herd-representation.rst

@@ -3,7 +3,7 @@
 herd-representation
 -------------------
 
-Literal include of ``tools/memory-model/Documentation/herd-representation``.
+Literal include of ``tools/memory-model/Documentation/herd-representation.txt``.
 
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