input: Write a function that takes a string and capitalizes the first character of each word.
  assert capitalize_string("this is an example")=="This Is An Example"
  assert capitalize_string("hello world")=="Hello World"
  assert capitalize_string("python programming")=="Python Programming"
  assert capitalize_string("capitalize every word")=="Capitalize Every Word"
  assert capitalize_string("a quick brown fox")=="A Quick Brown Fox"
output: def capitalize_string(s):
      return ' '.join(word.capitalize() for word in s.split())