Left-over "D" for the mouth diameter, in the documentation

That's a tiny detail, but there is still a little D that actually means "mouth diameter" in the short-range documentation (left-over from MR !407 (merged)), while D normally should mean "particle diameter". This is when t0 is defined (see https://gitlab.cern.ch/cara/caimira/-/blob/master/caimira/docs/full_diameter_dependence.rst?plain=1#L148).