logger: improve logger --journald man page example

logger: improve logger --journald man page example

The example use of logger --journald in the man page has a couple of flaws:
 - It's missing a "MESSAGE=" field. This is supposed to be the primary
   human readable text. Without it the log entry is invisible in a
   plain "journalctl" output.
 - The MESSAGE_ID is supposed to be a 128-bit hexadecimal string that
   globally uniquely identifies the message type.
   One can generate such an id with "journalctl --new-id".

This patches fixes the above and also changes the example to use a
here-document instead of printf. In my opinion it makes the expected
multi-line data format more obvious.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>