PWGEM: fix preslice error with PresliceUnsorted (#2737)

Author: feisenhu

PWGEM/Dilepton/Tasks/MCtemplates.cxx

@@ -509,7 +509,7 @@ struct AnalysisSameEventPairing {
     } // end of true pairing loop
   }   // end runMCGen
 
-  Preslice<ReducedMCTracks> perReducedMcEvent = aod::reducedtrackMC::reducedMCeventId;
+  PresliceUnsorted<ReducedMCTracks> perReducedMcEvent = aod::reducedtrackMC::reducedMCeventId;
 
   void processDecayToEESkimmed(soa::Filtered<MyEventsSelected>::iterator const& event,
                                soa::Filtered<MyBarrelTracksSelected> const& tracks,