input: Write a function to filter strings containing the letter 'a' using list comprehension.
  assert filter_strings(['ant', 'bar', 'cat', 'dog', 'eel']) == ['ant', 'bar', 'cat']
  assert filter_strings(['apple', 'banana', 'cherry', 'date', 'fig']) == ['apple', 'banana', 'date']
  assert filter_strings(['grape', 'kiwi', 'mango', 'pear', 'plum']) == ['grape', 'mango', 'pear']
output: def filter_strings(strings):
      return [s for s in strings if 'a' in s]