docs: dt: Use full path to enable cross-reference

Use full path to reference writing-schema.rst so that it becomes a
cross-reference.
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@protonmail.com>
Link: https://lore.kernel.org/r/20201123121333.59274-1-nfraprado@protonmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/devicetree/bindings/submitting-patches.rst

@@ -25,7 +25,8 @@ I. For patch submitters
 
        make dt_binding_check
 
-     See ../writing-schema.rst for more details about schema and tools setup.
+     See Documentation/devicetree/writing-schema.rst for more details about
+     schema and tools setup.
 
   3) DT binding files should be dual licensed. The preferred license tag is
      (GPL-2.0-only OR BSD-2-Clause).