corrected typo on readme

Author: alessioricco

README.md

@@ -38,7 +38,7 @@ data = [('John Doe', 25, 50),
         ('Olivia Jackson', 29, 55)]
 df = pd.DataFrame(data, columns=['name', 'age', 'donation'])
 
-conv_df = PandasLLM(data=df, openai_api_key = os.environ.get("OPENAI_API_KEY"))
+conv_df = PandasLLM(data=df, llm_api_key = os.environ.get("OPENAI_API_KEY"))
 result = conv_df.prompt("What is the average donation of people older than 30 who donated more than $50?")
 
 print(f"Result ({type(result)}):\n {result}")