demo(slide-toggle): change outdated switch selector (#4141)

* At some point the `slide-toggle` has been called `switch`. The
selector inside of the demo-app is still using `switch` instead of
`slide-toggle`.

src/demo-app/slide-toggle/slide-toggle-demo.ts

@@ -3,7 +3,7 @@ import {Component} from '@angular/core';
 
 @Component({
   moduleId: module.id,
-  selector: 'switch-demo',
+  selector: 'slide-toggle-demo',
   templateUrl: 'slide-toggle-demo.html',
   styleUrls: ['slide-toggle-demo.css'],
 })