fix #1 CLI syntax

Usage: jsoncppgenerator.py [options]

Options:
  -h, --help            show this help message and exit
  --jsondatafile=JSONDATAFILE
                        required, JSON data file path
  --dstdir=DSTDIR       directory to save the generated code files, default is
                        current directory
  --gentest             generate test code "main.cpp", default is false