user: I was discussing movies with my friend last night, and we started talking about "Avatar." I realized I don't remember who directed it. Can you find out the director's name for me?

Available functions:
name: get_movie_director
description: Fetches the director of a movie from the OMDB API.
parameters: type: dict
properties: movie_name: type: string
description: The name of the movie.
required: movie_name