fix binning of the vertex histograms to fit integer values correctly

fix a spike in the distribution of number of reconstructed vertices observed in the 10th round of AthenaMT validation https://indico.cern.ch/event/985167/contributions/4148735/attachments/2169676/3664177/sutt-val-2021-01-13.pdf https://its.cern.ch/jira/browse/ATR-22441 ATR-22441 It is not a real effect but a consequence of the histogram binning Tagging @sutt , @nagano , @okumura