input: Write a python function to prompt the user to enter a name and print a greeting.
  assert greet_user() == "Hello John"  # Input: "John"
  assert greet_user() == "Hello Alice"  # Input: "Alice"
  assert greet_user() == "Hello Bob"  # Input: "Bob"
output: def greet_user():
      name = input("Enter your name: ")
      greeting = f"Hello {name}"
      print(greeting)
      return greeting