fix crossreference botch

Reference: /n/atom/patch/applied2013/pxeofmannit
Date: Fri Jun 14 17:12:54 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/man/8/pxeof	Fri Jun 14 17:12:44 2013
+++ /sys/man/8/pxeof	Fri Jun 14 17:12:45 2013
@@ -6,7 +6,7 @@
 .I system...
 .SH DESCRIPTION
 .I Pxeof uses
-.I ndb (6)
+.IR ndb (6)
 database to map system names to  a set of PXE files in
 .BR /cfg/pxe 
 by using system's