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>

misc-utils/logger.1

@@ -81,7 +81,13 @@ for details.  The use of a MESSAGE_ID field is generally a good idea, as it
 makes finding entries easy.  Examples:
 .IP
 .nf
-\fB    printf \(dq%s\\n%s\\n%s\\n\(dq MESSAGE_ID=42 DOGS=bark \(dqCARAVAN=goes on\(dq | logger --journald
+\fB    logger --journald <<end
+\fB    MESSAGE_ID=67feb6ffbaf24c5cbec13c008dd72309
+\fB    MESSAGE=The dogs bark, but the caravan goes on.
+\fB    DOGS=bark
+\fB    CARAVAN=goes on
+\fB    end
+.IP
 \fB    logger --journald=entry.txt
 .fi
 .IP