Update the documentation for running dita-topic

[jhradilek]

The dita-topic converter for Asciidoctor has recently implemented a command-line interface. This pull request updates the related instructions to reflect the intended way these tools are meant to be used together.

Implementation checklist

• The code changes come with the corresponding test cases
• The code changes pass all tests (run python3 -m unittest in the project directory)
• The code changes come with appropriate documentation