Don't use absolute path to sed when building usr.bin/join

This is required to build sort on Linux hosts since sed is in /bin there.

Approved By:	jhb (mentor)

usr.bin/sort/Makefile

@@ -7,7 +7,7 @@ PROG=	sort
 SRCS=	bwstring.c coll.c file.c mem.c radixsort.c sort.c vsort.c
 
 sort.1: sort.1.in
-	/usr/bin/sed ${MAN_SUB} ${.ALLSRC} >${.TARGET}
+	sed ${MAN_SUB} ${.ALLSRC} >${.TARGET}
 
 CLEANFILES+= sort.1