Rename to multiplicityPt.cxx and update implementation

Author: Paola Vargas Torres

PWGLF/Tasks/Nuspex/CMakeLists.txt

@@ -181,7 +181,7 @@ o2physics_add_dpl_workflow(chargedparticle-raa
     COMPONENT_NAME Analysis)
 
 o2physics_add_dpl_workflow(multiplicity-pt
-                    SOURCES MultiplicityPt.cxx
+                    SOURCES multiplicityPt.cxx
                     PUBLIC_LINK_LIBRARIES O2::Framework O2Physics::AnalysisCore
                     COMPONENT_NAME Analysis)