GFS2: docs update

Now http://sources.redhat.com/cluster/ is redirected to
http://sources.redhat.com/cluster/wiki/

Also fixed tabs in the end.
Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

Documentation/filesystems/gfs2.txt

 Global File System
 ------------------
 
-http://sources.redhat.com/cluster/
+http://sources.redhat.com/cluster/wiki/
 
 GFS is a cluster file system. It allows a cluster of computers to
 simultaneously use a block device that is shared between them (with FC,