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

Documentation/admin-guide/ext4.rst

@@ -392,7 +392,7 @@ When mounting an ext4 filesystem, the following option are accepted:
 
   dax
         Use direct access (no page cache).  See
-        Documentation/filesystems/dax.txt.  Note that this option is
+        Documentation/filesystems/dax.rst.  Note that this option is
         incompatible with data=journal.
 
   inlinecrypt

Documentation/filesystems/ext2.rst

@@ -25,7 +25,7 @@ check=none, nocheck	(*)	Don't do extra checking of bitmaps on mount
 				(check=normal and check=strict options removed)
 
 dax				Use direct access (no page cache).  See
-				Documentation/filesystems/dax.txt.
+				Documentation/filesystems/dax.rst.
 
 debug				Extra debugging information is sent to the
 				kernel syslog.  Useful for developers.