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.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>