input: Write a python function to count the occurrences of a specific letter in a given string.
  assert count_letter('s', 'Mississippi') == 4
  assert count_letter('i', 'Mississippi') == 4
  assert count_letter('p', 'Mississippi') == 2
  assert count_letter('m', 'Mississippi') == 1
output: def count_letter(letter, string):
      return string.lower().count(letter.lower())