Avoid using 'head' in generating groff doc date

It may not be available in certain cross build cases.

Note that this is a slight change in functionality, in that now only the
first line of the source ChangeLog file is processed. This is acceptable
as groff will be retired and we won't encounter a possibly-different
ChangeLog format.

Reported by:	jhibbits
Tested by:	jhibbits

gnu/usr.bin/groff/mdate.sh

@@ -4,6 +4,5 @@
 set -e
 test -r "$1"
 export LC_ALL=C
-changelog_date=$(sed -E -n 's/^([0-9]{4}-[0-9]{2}-[0-9]{2}).*$/\1/p' "$1" |\
-    head -n 1)
+changelog_date=$(sed -E -n '1s/^([0-9]{4}-[0-9]{2}-[0-9]{2}).*$/\1/p' "$1")
 echo $(date -j -f %Y-%m-%d +"%e %B %Y" $changelog_date)