Update the simtime parameter name: using_sim_time -> use_sim_time (#180)
Update the simtime parameter name: using_sim_time -> use_sim_time (#180)

* Correct simtime parameter name

Same as used in rclcpp currently and ros1

* Explain RTF a bit

* Move 'not real-time computing' higher up

Likely to be of interest to the reader at the beginning of the article

* copyedit

ros2/design,ros2/design,gbiggs/design,gbiggs/design,gbiggs/design,gbiggs/design,gbiggs/design,ros2/design,ros2/design,ros2/design