input: Write a python function to check the spelling of a word.
  assert spell_checker('Apple', ['apple', 'banana', 'orange']) == True
  assert spell_checker('Baboon', ['apple', 'banana', 'orange']) == False
  assert spell_checker('orange', ['apple', 'banana', 'orange']) == True
output: def spell_checker(word, word_list):
      return word.lower() in [w.lower() for w in word_list]