input: Write a python function to greet a user by their name, using sys.argv to take the name as an input.
  assert greet_user(["script_name.py", "Alice"]) == "Hello Alice"
  assert greet_user(["script_name.py", "Bob"]) == "Hello Bob"
  assert greet_user(["script_name.py", "John"]) == "Hello John"
output: import sys
  
  def greet_user(argv):
      name = argv[1]
      return f"Hello {name}"