md5.1: Fix the GNU mode example when using a digest file

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, des
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D44098