docs(slider): change slider value binding to ngModel (#8338)

Author: alekseyzadorozhniy

src/material-examples/slider-configurable/slider-configurable-example.html

@@ -56,7 +56,7 @@ <h2 class="example-h2">Result</h2>
         [step]="step"
         [thumb-label]="thumbLabel"
         [tick-interval]="tickInterval"
-        [value]="value"
+        [(ngModel)]="value"
         [vertical]="vertical">
     </mat-slider>
   </mat-card-content>