input: Write a python function to count the number of occurrences of the character 'a' in a given string.
  assert count_a("abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabc") == 17
  assert count_a("a quick brown fox jumps over a lazy dog") == 2
  assert count_a("") == 0
output: def count_a(s):
      return s.count('a')