input: Write a function to replace a given word in a string with another word. The function should take three parameters: a sentence, the old word, and the new word. It should return the modified sentence.
  assert replace_word("I love walking on the beach", "walking", "swimming") == "I love swimming on the beach"
  assert replace_word("Python is fun", "fun", "fascinating") == "Python is fascinating"
  assert replace_word("Coffee is my favorite drink", "Coffee", "Tea") == "Tea is my favorite drink"
  assert replace_word("Hello world", "world", "universe") == "Hello universe"
  assert replace_word("Empty string check", "check", "") == "Empty string "
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)