Fix magnetic field vector in em_shower and vertical_EAS examples

The definition of the MagneticFieldVector in the em_shower and vertical_EAS examples pretends to be Earth-like (and CORSIKA7-like). However, we messed up the sign of the z-component 😄

You can do the cross-check by printing out the magnetic field we use in corsika.cpp, i.e. wmm.getField(2022.5, 10_km, 49, 8.4), which prints out (2.05579e-05 -1.12474e-06 -4.3864e-05)