Add time unit to Limit#onSample

[Meijuh]

This simple commit adds the proper time unit (nanos) to the Javadoc for Limit#onSample.

I made a mistake by supplying milliseconds; I hope this prevents others from doing the same.

This PR could be extended in two ways:

1. Suffix all variables throughout the code base with the required time unit, e.g., Limit#onSample(long startTimeNanos, long rttNanos, int inflight, boolean didDrop); and many more places (where no TimeUnit parameter is given).
2. Run Checker Framework's Units checker on the code base: https://checkerframework.org/manual/#units-annotations.