input: Write a function to replace a given word in a string with another word. The function should take three parameters: the original string, the word to be replaced, and the word to replace it with. The function should return the modified string.
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("Hello world", "world", "earth")=="Hello earth"
  assert replace_word("Let's go for a run", "run", "walk")=="Let's go for a walk"
  assert replace_word("Python is fun", "fun", "awesome")=="Python is awesome"
  assert replace_word("I love coding", "coding", "programming")=="I love programming"
output: def replace_word(original_string, word_to_replace, replacement_word):
      return original_string.replace(word_to_replace, replacement_word)