input: Write a python function to count the elements in a list that contain a specific letter.
  assert count_elements_with_letter(['cat','dog','fish','apple'], 'a') == 2
  assert count_elements_with_letter(['banana','apple','grape','kiwi'], 'e') == 2
  assert count_elements_with_letter(['table','chair','desk','lamp'], 'x') == 0
output: def count_elements_with_letter(lst, letter):
      return sum(1 for word in lst if letter in word)