Documentation: kcsan: fix "Plain Accesses and Data Races" URL in kcsan.rst

Make the URL point to the "Plain Accesses and Data Races" section again
and prevent it from becoming stale by adding a commit id to it.

Signed-off-by: Ignacio Encinas <ignacio@iencinas.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250306-fix-plain-access-url-v1-1-9c653800f9e0@iencinas.com

Author: IEncinas10

Documentation/dev-tools/kcsan.rst

@@ -203,7 +203,7 @@ they happen concurrently in different threads, and at least one of them is a
 least one is a write. For a more thorough discussion and definition, see `"Plain
 Accesses and Data Races" in the LKMM`_.
 
-.. _"Plain Accesses and Data Races" in the LKMM: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt#n1922
+.. _"Plain Accesses and Data Races" in the LKMM: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt?id=8f6629c004b193d23612641c3607e785819e97ab#n2164
 
 Relationship with the Linux-Kernel Memory Consistency Model (LKMM)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~