input: Write a python function to count the occurrences of a specific letter in a given string.
  assert CountLetterOccurrences("hello world", "l") == 3
  assert CountLetterOccurrences("banana", "a") == 3
  assert CountLetterOccurrences("mississippi", "s") == 4
output: def CountLetterOccurrences(s: str, letter: str) -> int:
      return s.count(letter)