input: Write a python function to concatenate the first letter of each string in a list.
  assert first_letter_concat(['apple', 'banana', 'cherry']) == 'abc'
  assert first_letter_concat(['dog', 'elephant', 'frog']) == 'def'
  assert first_letter_concat(['grape', 'hippo', 'iguana']) == 'ghi'
output: def first_letter_concat(lst):
      return ''.join(word[0] for word in lst)