Correct readme example code

[jcbhmr]

Right now it says:

import emptyStack from '@iter-tools/immutable-stack'; // OR
const emptyStack = require('@iter-tools/immutable-stack');

and

import { ImmutableStackFrame } from '@iter-tools/immutable-stack'; // OR
const ImmutableStackFrame = require('@iter-tools/immutable-stack').constructor;

When the https://www.npmjs.com/package/@iter-tools/immutable-stack package doesn't exist. Only https://www.npmjs.com/package/@iter-tools/imm-stack exists.

It'd probably be a good idea to update that... 😉