doc/filters: remove double quotes from zoompan example

Based on suggested change by Clément Bœsch.

Signed-off-by: Lou Logan <lou@lrcd.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
cmdutils: implement FFREPORT=level=...

Fixes Ticket3057

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>