input: Write a python function to manipulate a list of strings so that each string contains only the first letter from the original string.
  assert shorten_string(["apple", "banana", "coconut"]) == ["a", "b", "c"]
  assert shorten_string(["dog", "elephant", "frog"]) == ["d", "e", "f"]
  assert shorten_string(["grape", "honeydew", "ice"]) == ["g", "h", "i"]
output: def shorten_string(strings):
      return [s[0] for s in strings]