input: Write a function to concatenate two given strings "string1" and "string2" with a space in between and print the result on the same line.
  assert print_hello_world()=="Hello World"
  assert print_hello_world()=="Hello World"
output: def print_hello_world():
      string1 = "Hello"
      string2 = "World"
      result = string1 + " " + string2
      return result