afs: Fix documentation on # vs % prefix in mount source specification

The documentation that describes the #-prefix and the %-prefix used when
specifying the source to mount is has the descriptions the wrong way
round.  Switch them over.
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>

Documentation/filesystems/afs.txt

@@ -91,8 +91,8 @@ Filesystems can be mounted anywhere by commands similar to the following:
 	mount -t afs "#root.cell." /afs/cambridge
 
 Where the initial character is either a hash or a percent symbol depending on
-whether you definitely want a R/W volume (hash) or whether you'd prefer a R/O
-volume, but are willing to use a R/W volume instead (percent).
+whether you definitely want a R/W volume (percent) or whether you'd prefer a
+R/O volume, but are willing to use a R/W volume instead (hash).
 
 The name of the volume can be suffixes with ".backup" or ".readonly" to
 specify connection to only volumes of those types.