input: Write a function to generate a Python program that takes a string and capitalizes the first character of each word. Return the capitalized string.
  assert capitalize_string("this is an example") == "This Is An Example"
  assert capitalize_string("hello world") == "Hello World"
  assert capitalize_string("openai gpt-3") == "Openai Gpt-3"
  assert capitalize_string("a simple test") == "A Simple Test"
  assert capitalize_string("") == ""
output: def capitalize_string(s):
      return ' '.join(word.capitalize() for word in s.split())