Make an example more illustrative in Generics.md

[grebenyuksv]

When reading this, I thought the authors' idea must have been to use different names in the generic type parameter and the type. Doesn't the suggested version illustrate the concept better? 🤔

We could also have used a different name for the generic type parameter in the type, so long as the number of type variables and how the type variables are used line up.

Before
The name is Input in both declarations:

function identity<Input>(arg: Input): Input {
  return arg;
}

let myIdentity: <Input>(arg: Input) => Input = identity;

After
The name is Type in the first declaration but Input in the second:

function identity<Type>(arg: Type): Type {
  return arg;
}

let myIdentity: <Input>(arg: Input) => Input = identity;

I don't think this PR suggests anything new — I rather believe the suggested version was the original intention.