dt-bindings: Fix reference in submitting-patches.rst to the DT ABI doc

submitting-patches.rst has a stale reference to ABI.txt. Fix this with a
proper rSt link.

Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/20210325164713.1296407-2-robh@kernel.org

Documentation/devicetree/bindings/submitting-patches.rst

@@ -84,7 +84,7 @@ II. For kernel maintainers
 III. Notes
 ==========
 
-  0) Please see ...bindings/ABI.txt for details regarding devicetree ABI.
+  0) Please see :doc:`ABI` for details regarding devicetree ABI.
 
   1) This document is intended as a general familiarization with the process as
      decided at the 2013 Kernel Summit.  When in doubt, the current word of the