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>