fix readme typo

README.md

@@ -114,7 +114,7 @@ You can also add your own custom tools for your vision agent to use:
 >>> }
 >>> ],
 >>> }
->>> def __call__(self, prompt: str) -> int:
+>>> def __call__(self, prompt: list[str]) -> int:
 >>> return len(prompt)
 ```
 This will register it with the list of tools Vision Agent has access to. It will be able