Fix typo in references.bib

doc/usermanual/references.bib

@@ -301,7 +301,7 @@ keywords = "Simulation, Silicon detectors, Geant4, TCAD, Drift–diffusion"
       url           = "https://cds.cern.ch/record/2649493",
       note          = {accessed 11~1019}
 }
-@online{root-tefficiency-class-reference,
+@online{root-tefficiency-class-ref,
     title = {ROOT TEfficiency Class Reference},
     author = {},
     url = {https://root.cern.ch/doc/master/classTEfficiency.html#ae80c3189bac22b7ad15f57a1476ef75b},