input: Write a function to replace all occurrences of the letter 'e' with '@' in each string of a list.
  assert replace_e(['hello', 'world', 'example']) == ['h@llo', 'world', '@xampl@']
  assert replace_e(['eagle', 'elephant', 'enter']) == ['@agl@', '@l@phant', '@nt@r']
  assert replace_e(['tree', 'free', 'see']) == ['tr@@', 'fr@@', 's@@']
output: def replace_e(lst):
      return [s.replace('e', '@') for s in lst]