examples: json-to-csv: fix invocation
examples: json-to-csv: fix invocation

The two filenames are actually mandatory, not optional.

Reported-by: 张路 <451036e39ef1d15306be02ad9b74b4a510cc539e@bupt.edu.cn>
Change-Id: I4e40748430a602a2c06eb35c96e0a8267519b8b3

Telecominfraproject/gnpy