input: Write a function to count the occurrences of a specific word in a given sentence.
  assert word_count("She sells sea shells on the sea shore", "sea") == 2
  assert word_count("How much wood would a woodchuck chuck if a woodchuck could chuck wood", "wood") == 2
  assert word_count("The quick brown fox jumps over the lazy dog", "the") == 2
output: def word_count(sentence, word):
      words = sentence.lower().split()
      return words.count(word.lower())