add formular in docu C_term + N_term + Hydrogen - Electron;

[cpanse]

https://github.com/protViz/protViz/blob/ebd3232ee4bf8415d8468b60ee59d66ee64b7cb1/man/parentIonMass.Rd#L11

C_term + N_term + Hydrogen - Electron;