input: Write a function to generate a Python program that takes a string and capitalizes the first character of each word in the string. The function should return the modified string.
  assert capitalize_string("this is an example")=="This Is An Example"
  assert capitalize_string("hello world")=="Hello World"
  assert capitalize_string("a quick brown fox")=="A Quick Brown Fox"
  assert capitalize_string("")==""
  assert capitalize_string("already capitalized")=="Already Capitalized"
output: def capitalize_string(s):
      return ' '.join(word.capitalize() for word in s.split())