message: docs(introduction): update examples
diff: @@ -19,14 +19,16 @@ The essential concepts in RxJS which solve async event management are:
  Normally you register event listeners.
  ```js
  -var button = document.querySelector('button');
  +const button = document.querySelector('button');
  button.addEventListener('click', () => console.log('Clicked!'));
  ```
  Using RxJS you create an observable instead.
  ```js
  -var button = document.querySelector('button');
  -Rx.Observable.fromEvent(button, 'click')
  +const { fromEvent } = rxjs;
  +
  +const button = document.querySelector('button');
  +fromEvent(button, 'click')
  .subscribe(() => console.log('Clicked!'));
  ```
  @@ -44,8 +46,11 @@ button.addEventListener('click', () => console.log(`Clicked ${++count} times`));
  Using RxJS you isolate the state.
  ```Js
  -var button = document.querySelector('button');
  -Rx.Observable.fromEvent(button, 'click').pipe(
  +const { fromEvent } = rxjs;
  +const { scan } = rxjs.operators;
  +
  +cosnt button = document.querySelector('button');
  +fromEvent(button, 'click').pipe(
  scan(count => count + 1, 0)
  )
  .subscribe(count => console.log(`Clicked ${count} times`));
  @@ -72,8 +77,11 @@ button.addEventListener('click', () => {
  With RxJS:
  ```js
  -var button = document.querySelector('button');
  -Rx.Observable.fromEvent(button, 'click').pipe(
  +const { fromEvent } = rxjs;
  +const { throttleTime, scan } = rxjs.operators;
  +
  +const button = document.querySelector('button');
  +fromEvent(button, 'click').pipe(
  throttleTime(1000),
  scan(count => count + 1, 0)
  )
  @@ -87,10 +95,10 @@ You can transform the values passed through your observables.
  Here's how you can add the current mouse x position for every click, in plain JavaScript:
  ```js
  -var count = 0;
  -var rate = 1000;
  -var lastClick = Date.now() - rate;
  -var button = document.querySelector('button');
  +let count = 0;
  +const rate = 1000;
  +let lastClick = Date.now() - rate;
  +const button = document.querySelector('button');
  button.addEventListener('click', (event) => {
  if (Date.now() - lastClick >= rate) {
  count += event.clientX;
  @@ -102,8 +110,11 @@ button.addEventListener('click', (event) => {
  With RxJS:
  ```js
  -var button = document.querySelector('button');
  -Rx.Observable.fromEvent(button, 'click').pipe(
  +const { fromEvent } = rxjs;
  +const { throttleTime, map, scan } = rxjs.operators;
  +
  +const button = document.querySelector('button');
  +fromEvent(button, 'click').pipe(
  throttleTime(1000),
  map(event => event.clientX),
  scan((count, clientX) => count + clientX, 0)
  