Fixed mdoc(7) warnings visible in troff mode only.

Spotted by:	Rich Morin <rdm@cfcl.com>

share/man/man8/intro.8

@@ -42,27 +42,27 @@
 This section contains information related to system operation
 and maintenance.
 It describes commands used to create new file systems,
-.Ql Xr newfs ,
+.Xr newfs 8 ,
 verify the integrity of the file systems,
-.Ql Xr fsck ,
+.Xr fsck 8 ,
 control disk usage,
-.Ql Xr edquota ,
+.Xr edquota 8 ,
 maintain system backups,
-.Ql Xr dump ,
+.Xr dump 8 ,
 and recover files when disks die an untimely death,
-.Ql Xr restore .
+.Xr restore 8 .
 The
-.Ql Xr format
+.Xr format 8
 manual
 for the specific architecture the system is running on should be
 consulted when formatting disks and tapes.
 Network related services like
-.Ql Xr inetd
+.Xr inetd 8
 and
-.Ql Xr ftpd
+.Xr ftpd 8
 are also described.
 The section
-.Ql Xr crash
+.Xr crash 8
 should be consulted to understand how to interpret system
 crash dumps.
 .Pp