Fix '*args' typing for Nested parameter: List[Parameter]->Parameter

Nested parameter has typing for argument list as:

class Nested(NestedType):
    ...
    def __init__(self, *parameters: List[Parameter]):
        self.parameters = parameters
    ...

Typing for argument list: https://www.python.org/dev/peps/pep-0484/#arbitrary-argument-lists-and-default-argument-values

Changed to:

class Nested(NestedType):
    ...
    def __init__(self, *parameters: Parameter):
        self.parameters = parameters
    ...