input: Write a python function to filter out strings without the letter 'a'.
  assert filter_strings_with_a(['apple', 'banana', 'grape', 'cherry', 'blueberry'])==['apple', 'banana', 'grape']
  assert filter_strings_with_a(['car', 'dog', 'cat', 'bird'])==['car', 'cat']
  assert filter_strings_with_a(['table', 'chair', 'desk'])==['table']
output: def filter_strings_with_a(strings):
      return [s for s in strings if 'a' in s]