input: Write a function to take a string and capitalize the first character of each word. If the string is empty, return the empty string.
  assert capitalize_string("this is an example")=="This Is An Example"
  assert capitalize_string("hello world")=="Hello World"
  assert capitalize_string("a b c d e")=="A B C D E"
  assert capitalize_string("")==""
output: def capitalize_string(s):
      return ' '.join(word.capitalize() for word in s.split())