input: Write a python function to replace the word "Hello" with "Goodbye" in a given string.
  assert replace_hello("Hello, World!") == "Goodbye, World!"
  assert replace_hello("Hello, Hello!") == "Goodbye, Goodbye!"
  assert replace_hello("Say Hello to the world") == "Say Goodbye to the world"
output: def replace_hello(input_string):
      return input_string.replace("Hello", "Goodbye")