input: Write a python function to count the occurrences of a keyword in a given sentence.
  assert count_keyword("This is a sample sentence for the purpose of the example. ", "example") == 1
  assert count_keyword("Repetition is key to learning. Repetition helps.", "Repetition") == 2
  assert count_keyword("No matching keyword here.", "missing") == 0
output: def count_keyword(sentence, keyword):
      return sentence.split().count(keyword)