chore(demo): make vertical nav collapsable (#1592)

## PR Checklist

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- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)
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## PR Type

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- [ ] Bugfix
- [ ] Feature
- [ ] Code style update (formatting, local variables)
- [x] Refactoring (no functional changes, no api changes)
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## What is the current behavior?

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link to a relevant issue. -->

Issue Number: N/A

## What is the new behavior?

## Does this PR introduce a breaking change?

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- [x] No

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## Other information

projects/demo/src/app/content-container.component.ts

@@ -13,7 +13,7 @@ import { APP_ROUTES } from './app.routing';
 @Component({
   selector: 'my-app-content-container',
   template: `
-    <clr-vertical-nav>
+    <clr-vertical-nav [clrVerticalNavCollapsible]="true">
       <ng-container *ngFor="let route of routes">
         <a
           *ngIf="route.path != ''"