devices.txt: fix typo of "ubd" as "udb"

Signed-off-by: Theodore Dubois <tblodt@icloud.com>
Link: https://lore.kernel.org/r/20200816233823.86316-1-tblodt@icloud.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/devices.txt

@@ -1662,7 +1662,7 @@
 
   98 block	User-mode virtual block device
 		  0 = /dev/ubda		First user-mode block device
-		 16 = /dev/udbb		Second user-mode block device
+		 16 = /dev/ubdb		Second user-mode block device
 		    ...
 
 		Partitions are handled in the same way as for IDE