fix typo in G4EmExtraPhysics
This MR fixes a bug in the EmExtraPhysics which has been fixed in Geant4 long time ago, but not backported to the LHCb/Geant4 repo.
It is a copy-paste typo where the electron-nuclear process is attached to the positrons. As there are more things like this, it would be extremely helpful to use a later Geant4 version, as proposed in !116