[sensors] Fix Sensor.start() operation description

pozdnyakov has just created a new issue for 
https://github.com/w3c/sensors:

== Fix Sensor.start() operation description ==
It has a logic error:

3. Set sensor_instance.[[state]] to "activating".
4.1. If sensor_state is "unconnected", then: 



Please view or discuss this issue at 
https://github.com/w3c/sensors/issues/161 using your GitHub account