md5.1: Fix an example

The last example in the manpage md5(1) wants to demonstrate
GNU mode (md5sum), but uses BSD mode (md5) instead:

In GNU mode, the -c option does not compare against a hash string
passed as parameter. Instead, it expects a  digest file,
as created under the name digest for /boot/loader.conf in
the example above.

PR:	276560
Reviewed by:	mhorne
Differential Revision:	https://reviews.freebsd.org/D44098

(cherry picked from commit 865baeaf1abeb14327ad6a4a1f8ce722e242ff73)

sbin/md5/md5.1

@@ -1,4 +1,4 @@
-.Dd February 13, 2024
+.Dd February 28, 2024
 .Dt MD5 1
 .Os
 .Sh NAME
@@ -367,7 +367,7 @@ for
 .Pa /boot/loader.conf
 in the example above.
 .Bd -literal -offset indent
-$ md5 -c digest /boot/loader.conf
+$ md5sum -c digest /boot/loader.conf
 /boot/loader.conf: OK
 .Ed
 .Pp