Fix observers section in Lit 2.1 release blog post (lit#933)

* fix observers code example in Lit 2.1 release post
* Fix plural form of package name

Author: AndrewJakubowicz

packages/lit-dev-content/site/blog/2022-01-05-lit-2.1-release.md

@@ -116,18 +116,20 @@ alternative to the positional argument API.
 private _icon!: Array<HTMLElement>;
 ```
 
-## Easy to use observers with @lit-labs/observer
+## Easy to use observers with @lit-labs/observers
 
-In Lit Labs we've introduced a new package called `@lit-labs/observer` which
+In Lit Labs we've introduced a new package called `@lit-labs/observers` which
 contains reactive controllers that let you easily use the web platform observer
 APIs: MutationObserver, IntersectionObserver, ResizeObserver, and
 PerformanceObserver. The controllers automatically drive the host element's
 update lifecycle when they observe changes.
 
 ```ts
+import { ResizeController } from '@lit-labs/observers/resize_controller.js';
+
 class MyResizableElement extends LitElement {
 
-  _resizeController = new ResizeController(this);
+  _resizeController = new ResizeController(this, {});
 
   render() {
     return html`Current size is ${this.offsetWidth} x ${this.offsetHeight}`;