Fix syntax error in NamedTuple example

$ python3 -c "typing.NamedTuple('NT', [('a': int, 'b': Tuple[str, ...])])"
  File "<string>", line 1
    typing.NamedTuple('NT', [('a': int, 'b': Tuple[str, ...])])
                                 ^
SyntaxError: invalid syntax