input: Write a python function to count the occurrences of a specific word in a given sentence.
  assert count_word("hello world hello everyone", "hello") == 2
  assert count_word("this is a test this is only a test", "test") == 2
  assert count_word("one fish two fish red fish blue fish", "fish") == 4
output: def count_word(sentence, word):
      words = sentence.split()
      return words.count(word)