cgitrc.5: tar.xz is a supported snapshot format

When tar.xz support was added in 0642435fed (2009-12-08: Add
.tar.xz-snapshot support), cgitrc.5 was not updated to match. This
patch fixes the issue.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgitrc.5.txt

@@ -308,7 +308,8 @@ side-by-side-diffs::
 snapshots::
 	Text which specifies the default set of snapshot formats generated by
 	cgit. The value is a space-separated list of zero or more of the
-	values "tar", "tar.gz", "tar.bz2" and "zip". Default value: none.
+	values "tar", "tar.gz", "tar.bz2", "tar.xz" and "zip". Default value:
+	none.
 
 source-filter::
 	Specifies a command which will be invoked to format plaintext blobs