Fix generic parameter defaults section in Generics.md

[Paweł Smolak (psmolak)]

Let's suppose that

class Animal {
  name: string = "TS";
}

then the first create declaration

declare function create(): Container<HTMLDivElement, HTMLDivElement[]>;
declare function create<T extends HTMLElement>(element: T): Container<T, T[]>;
declare function create<T extends HTMLElement, U extends HTMLElement>(
  element: T,
  children: U[]
): Container<T, U[]>;

correctly catches create(new HTMLElement(), [new Animal()]) as type error, whereas the shortened version, that is

declare function create<T extends HTMLElement = HTMLDivElement, U = T[]>(
  element?: T,
  children?: U
): Container<T, U>;

does allow create(new HTMLElement(), [new Animal()]).

[Jake Bailey (jakebailey)]

I personally find this example odd due to the default being T[] (why would the children be the same type as the parent?) but that's preexisting and the PR is correct to my eye.