Remove `option` parameter for `AIChat.__str__`

[minimaxir]

{
    "name": "TypeError",
    "message": "BaseModel.model_dump_json() got an unexpected keyword argument 'option'",
    "stack": "---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[36], line 1
----> 1 print(ai)

File /opt/homebrew/lib/python3.11/site-packages/simpleaichat/simpleaichat.py:220, in AIChat.__str__(self)
    218 def __str__(self) -> str:
    219     if self.default_session:
--> 220         return self.default_session.model_dump_json(
    221             exclude={\"api_key\", \"api_url\"},
    222             exclude_none=True,
    223             option=orjson.OPT_INDENT_2,
    224         )

TypeError: BaseModel.model_dump_json() got an unexpected keyword argument 'option'"
}

Should have been removed in the Pydantic 2.0 migration.