input: Write a function to replace a given word in a string with another word. The function should accept three parameters: the original string, the word to be replaced, and the new 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("Python is great", "great", "awesome")=="Python is awesome"
  assert replace_word("I love coding in Python", "coding", "programming")=="I love programming in Python"
  assert replace_word("This is a test", "test", "example")=="This is a example"
  assert replace_word("", "test", "example")==""
output: def replace_word(original_string, word_to_replace, new_word):
      return original_string.replace(word_to_replace, new_word)