input: Write a python function to search for a word in a dictionary.
  assert search_word('hello', {'hello': 'string', 'goodbye': 'bye'}) == 'string'
  assert search_word('goodbye', {'hello': 'string', 'goodbye': 'bye'}) == 'bye'
  assert search_word('welcome', {'hello': 'string', 'goodbye': 'bye'}) == None
output: def search_word(word, dictionary):
      return dictionary.get(word)