Improve sample codes about json.dumps usage

[peter279k]

With your PR, here is a check list:

• Has test cases written?
• Has all code lines tested?
• Has make format been run?
• Please update CHANGELOG.yml(not CHANGELOG.rst)
• Has fair amount of documentation if your change is complex
• Agree on NEW BSD License for your contribution

Sometimes it will throw the "datetime.datetime not JSON serializable"? exception error when using the json.dumps method.
To avoid throwing the above exception message, it should add the default parameter with the str value. And it can be great to use json.dumps method without throwing the exception message.