input: Write a python function to count the occurrences of a specific character in a string.
  assert count_characters("abracadabra", 'a') == 5
  assert count_characters("hello world", 'o') == 2
  assert count_characters("mississippi", 's') == 4
output: def count_characters(s: str, char: str) -> int:
      return s.count(char)