input: Write a python function to concatenate two strings.
  assert concatenate_two_strings("Hello", " World") == "Hello World"
  assert concatenate_two_strings("Good", "Morning") == "GoodMorning"
  assert concatenate_two_strings("Python", "Rocks") == "PythonRocks"
output: def concatenate_two_strings(str1, str2):
      return str1 + str2