docs: fix locations of several documents that got moved

The previous patch renamed several files that are cross-referenced
along the Kernel documentation. Adjust the links to point to
the right places.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
diff --git a/Documentation/isdn/README b/Documentation/isdn/README
index cfb18843..32d4e80 100644
--- a/Documentation/isdn/README
+++ b/Documentation/isdn/README
@@ -332,7 +332,7 @@
 4. Device-inodes
 
    The major and minor numbers and their names are described in
-   Documentation/devices.txt. The major numbers are:
+   Documentation/admin-guide/devices.rst. The major numbers are:
 
      43 for the ISDN-tty's.
      44 for the ISDN-callout-tty's.