input: Write a python function to find the index of a string in a list.
  assert find_string('apple', ['banana', 'cherry', 'apple', 'date']) == 2
  assert find_string('cherry', ['banana', 'cherry', 'apple', 'date']) == 1
  assert find_string('date', ['banana', 'cherry', 'apple', 'date']) == 3
  assert find_string('banana', ['banana', 'cherry', 'apple', 'date']) == 0
  assert find_string('fig', ['banana', 'cherry', 'apple', 'date']) == None
output: def find_string(target, string_list):
      try:
          return string_list.index(target)
      except ValueError:
          return None