Recommended docker image remove syntax

[pjamason]

File: engine/getstarted/step_six.md, CC: @mstanleyjones

docker remove should be docker rmi, correct?

[shubheksha]

This has been fixed, this issue can be closed.

[mdlinville]

Make note of your points for closing it!

[shubheksha]

Yes, I'll track them in a spreadsheet, thanks for the tip! :)