Fix for physics validation HT TRT plots using only Xenon hits

Deceptively, numberOfTRTHighThresholdHits refer to the number of Xenon hits, numberOfTRTHighThresholdHitsTotal should be used to get the total number of hits. This one-line changes makes sure the Physics Validation plots will use the latter. I don't expect frozen T0 tests to check physics validation histograms, so they should pass.