xfs: Change URL for the project in xfs.txt

The oss.sgi.com doesn't exist any more.
Change it to current project URL, https://xfs.wiki.kernel.org/Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/xfs.txt

@@ -9,7 +9,7 @@ variable block sizes, is extent based, and makes extensive use of
 Btrees (directories, extents, free space) to aid both performance
 and scalability.
 
-Refer to the documentation at http://oss.sgi.com/projects/xfs/
+Refer to the documentation at https://xfs.wiki.kernel.org/
 for further details.  This implementation is on-disk compatible
 with the IRIX version of XFS.