fix(progressbar): add missing i18n id specifier

The id was picked up as description by the angular cli, which resulted in a random id.

Closes #2493

src/progressbar/progressbar.ts

@@ -13,7 +13,7 @@ import {NgbProgressbarConfig} from './progressbar-config';
       <div class="progress-bar{{type ? ' bg-' + type : ''}}{{animated ? ' progress-bar-animated' : ''}}{{striped ?
     ' progress-bar-striped' : ''}}" role="progressbar" [style.width.%]="getPercentValue()"
     [attr.aria-valuenow]="getValue()" aria-valuemin="0" [attr.aria-valuemax]="max">
-        <span *ngIf="showValue" i18n="ngb.progressbar.value">{{getPercentValue()}}%</span><ng-content></ng-content>
+        <span *ngIf="showValue" i18n="@@ngb.progressbar.value">{{getPercentValue()}}%</span><ng-content></ng-content>
       </div>
     </div>
   `