Docstring corrections and suggestion

[guiyrt]

• Correction in differential_cross_section docstring, adding term $\frac{\rho_\mu}{\rho_e}$
• Correction in minkowski_dot docstring, + signal replaced with -
• Suggestion based on notebook1 content, adding more context in coords_to_dict docstring. As a non-physicist, it was already somewhat difficult to get there with the added context 😁

Great work on the material!! 👍

[aoanla]

Just my 2¢/€, I agree with these changes - I have a list of places that need more context from discussions during the session (including with @guiyrt ) which includes all of these and a few more that I missed from the notebooks.