input: Write a python function to replace all occurrences of a word in a string with another word.
  assert replace_all_occurrences('Course: Machine Learning, Platform: Coursera.', 'Course', 'Program') == 'Program: Machine Learning, Platform: Coursera.'
  assert replace_all_occurrences('Hello World! Hello Universe!', 'Hello', 'Hi') == 'Hi World! Hi Universe!'
  assert replace_all_occurrences('The quick brown fox jumps over the lazy dog', 'fox', 'cat') == 'The quick brown cat jumps over the lazy dog'
output: def replace_all_occurrences(text, old_word, new_word):
      return text.replace(old_word, new_word)