input: Write a python function to replace the letter 'a' with '$' in a given string.
  assert replace_letter('a cat on a mat') == '$ c$t on $ m$t'
  assert replace_letter('banana') == 'b$n$n$'
  assert replace_letter('apple') == '$pple'
output: def replace_letter(s: str) -> str:
      return s.replace('a', '$')