Add data content type and encoding example in d2c command help

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.

Thank you for contributing to the IoT extension!

This checklist is used to make sure that common guidelines for a pull request are followed.

Intent for Production

[ ] It is expected that pull requests made to default or core branches such as dev or main are of production grade. Corollary to this, any merged contributions to these branches may be deployed in a public release at any given time. By checking this box, you agree and commit to the expected production quality of code.

Basic expectations

[ ] If introducing new functionality or modified behavior, are they backed by unit and/or integration tests?
[ ] In the same context as above are command names and their parameter definitions accurate? Do help docs have sufficient content?
[ ] Have all the relevant unit and integration tests pass? i.e. pytest <project root> -vv. Please provide evidence in the form of a screenshot showing a succesful run of tests locally OR a link to a test pipeline that has been run against the change-set.
[ ] Have linter checks passed using the .pylintrc and .flake8 rules? Look at the CI scripts for example usage.
[ ] Have extraneous print or debug statements, commented out code-blocks or code-statements (if any) been removed from the surface area of changes?
[ ] Have you made an entry in HISTORY.rst which concisely explains your user-facing feature or change?

Azure IoT CLI maintainers reserve the right to enforce any of the outlined expectations.

A PR is considered ready for review when all basic expectations have been met (or do not apply).

Azure / azure-iot-cli-extension