renaming VAR for multiline option (-b | --big-prompt) option and its …

[JulianvonGebhardi]

…fix to prevent error: UNKNOWN PARAMETER

I found a little issue with the multiline option. The option -b (--big-prompt) produced the error: "Unknown parameter". I quickly fixed it and renamed the var to match the options in your README.md for clarification.

And props! I really it!

[JulianvonGebhardi]

Since I just found the previous commit related to this. Another soltuion might be:

• keeping the naming "multi_line_prompt" and using -M to distinguish the -m (model) option.
  But it needed a fix anyway since the code did not comply with the stated option -b | --big-prompt in the README file. Now it works as intended.