docs: fix a cross-ref

Commit acda97ac changes dax.txt to dax.rst.
Fix the references accordingly.

Cc: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Link: https://lore.kernel.org/r/20210611030044.1982911-4-kolyshkin@gmail.com


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