Reminder to fix API example for mutual_spherical_harmonic_gravity

[minervino96]

The third-body acceleration example for the mutual_spherical_harmonic_gravity function states:
"we consider the spherical harmonic gravity acceleration mutually exerted between Ganymede and Io when propagating w.r.t. Jupiter"
However, the example code is:

# Create acceleration dict
acceleration_settings_on_io = dict()
# Add the acceleration to the dict
acceleration_settings_on_io["Jupiter"] = [propagation_setup.acceleration.mutual_spherical_harmonic_gravity(
     maximum_degree_of_jupiter,
     maximum_order_of_jupiter,
     maximum_degree_of_ganymede,
     maximum_order_of_ganymede,
     maximum_degree_of_io,
     maximum_order_of_io)]

While it should be:

# Create acceleration dict
acceleration_settings_on_io = dict()
# Add the acceleration to the dict
acceleration_settings_on_io["Ganymede"] = [propagation_setup.acceleration.mutual_spherical_harmonic_gravity(
     maximum_degree_of_ganymede,
     maximum_order_of_ganymede,
     maximum_degree_of_io,
     maximum_order_of_io,
     maximum_degree_of_jupiter,
     maximum_order_of_jupiter)]