Fix error in iallocator documentation reg. disk mode

The code uses the disk object's “mode” attribute, which uses the
constants DISK_RDONLY (“ro”) and DISK_RDWR (“rw”).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

doc/iallocator.rst

@@ -122,7 +122,7 @@ request
     instance (in the order they are exported to the hypervisor):
 
     mode
-      either ``r`` or ``w`` denoting if the disk is read-only or
+      either ``ro`` or ``rw`` denoting if the disk is read-only or
       writable
 
     size