Fix incorrect type annotations in get_writer function in utils.py

[JulianFP]

I noticed the following issue while trying to use WhisperX's output writing capabilities from within python code for my own project called Project-W. It is not exactly major however it results in my IDE complaining about incorrect types when calling the output of get_writer with a string as the second attribute. If I pass a TextIO instead as the type annotation suggests it fails. Here the more in-depth explanation:

The get_writer function returns a Callable which is currently annotated as follows: Callable[[dict, TextIO, dict], None].
get_writer then returns instances of writers (e.g. WriteJSON, WriteSRT) which all inherit from ResultWriter. The __call__ method of ResultWriter however is annotated like this: def __call__(self, result: dict, audio_path: str, options: dict):. These two annotations contradict each other because the __call__ method takes a str as the second argument while get_writer claims to return a callable which takes a TextIO as the second argument.

The type annotation of the __call__ method of ResultWriter is correct since the method expects a string to construct the os path, hence this PR adjusts the annotation in get_writer to fit the one of ResultWriter.