Hi Lino
This value is to be found in /administrator/language/en-GB/en-GB.plg_user_jopensimregister.ini for the variable PLG_JOPENSIMREGISTER_OPENSIM_DETAILS. But if you change it there, it will get overwritten with the next update again.
Better is to add a line in /language/overrides/en-GB.override.ini like:
PLG_JOPENSIMREGISTER_OPENSIM_DETAILS="Account Details for CoolWorldName"
Like this it will remain even after an update of jOpenSim.
If this file does not exist yet and you need to create it, be sure to save the file in utf-8 no bom
Greetz
FoTo50