Fix TrackParticleMerger properties
Aligning the property name to match with what is actually in the algorithm (https://gitlab.cern.ch/atlas/athena/-/blob/main/PhysicsAnalysis/Algorithms/TrackingAnalysisAlgorithms/TrackingAnalysisAlgorithms/TrackParticleMergerAlg.h?ref_type=heads#L60) + fix a property spelling which has been bothering me for years.