docs: fix a cross-ref

Commit acda97acb2e98c9 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>
diff --git a/Documentation/filesystems/ext2.rst b/Documentation/filesystems/ext2.rst
index c2fce22..154101c 100644
--- a/Documentation/filesystems/ext2.rst
+++ b/Documentation/filesystems/ext2.rst
@@ -25,7 +25,7 @@
 				(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.