input: Write a python function to interpolate a string by replacing placeholder {{name}} with the word "world".
  assert interpolate("Hello {{name}}") == "Hello world"
  assert interpolate("Welcome {{name}}, have a great day!") == "Welcome world, have a great day!"
  assert interpolate("Goodbye {{name}}") == "Goodbye world"
output: def interpolate(s: str) -> str:
      return s.replace("{{name}}", "world")