reduced uncertainty on QCD normalization in tautau channel

Changed
  CMS_htt_QCDSyst_tautau = 0.35
to
  CMS_htt_QCDSyst_tautau = 0.10
after discussion with Riccardo and Aram:
the QCD yield uncertainty is now handled via the QCDfrNorm and QCDfrShape nuissance parameters,
so it is justified to reduce the CMS_htt_QCDSyst_tautau nuissance parameter to 10%, to take any eventual dependence of the event category on the Tight/Loose jet -> tau fake-rate into account (10% = value used in etau and mutau channels)