[webrtc-stats] description of when roundTripTimeMeasurements increments is wrong (#515)

fippo has just created a new issue for https://github.com/w3c/webrtc-stats:

== description of when roundTripTimeMeasurements increments is wrong ==
https://w3c.github.io/webrtc-stats/#dom-rtcremoteinboundrtpstreamstats-roundtriptimemeasurements
says
```
This counter will increment if the roundTripTime is undefined. 
```
The opposite is true. Either:
```
This counter will not increment if the roundTripTime is undefined.
```
or
```
This counter will increment if the roundTripTime is not undefined. 
```

No preference in either direction :-)

Please view or discuss this issue at https://github.com/w3c/webrtc-stats/issues/515 using your GitHub account