Update make helpfile

Corrected typo, included the option dump-deps-png because that is
awesome and available and not in this helpfile! :-)

Change-Id: If8dd8b9c4dc9938c65499ae5d6dfdd7bf830e94d
Reviewed-on: https://gerrit.libreoffice.org/8747
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>