batman-adv: Switch mailing list subscription page

The mailman installation on lists.open-mesh.org was switched from mailman2
to mailman3. The URL to the subscription webpage changed in this process.
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

Documentation/networking/batman-adv.rst

@@ -160,7 +160,7 @@ IRC:
   #batman on irc.freenode.org
 Mailing-list:
   b.a.t.m.a.n@open-mesh.org (optional subscription at
-  https://lists.open-mesh.org/mm/listinfo/b.a.t.m.a.n)
+  https://lists.open-mesh.org/mailman3/postorius/lists/b.a.t.m.a.n.lists.open-mesh.org/)
 
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