docs(material/slider): changed value property of mat-slider from gett… (#23276)

The mat-slider is invisible when value is 1 on the purple-green  pre-built template after following the "getting started" guide. Since the white background is default nothing can be seen, which is especially bad for newbies.

(cherry picked from commit 74f18ed)

Author: GuilhermeRossato

guides/getting-started.md

@@ -62,7 +62,7 @@ import { MatSliderModule } from '@angular/material/slider';
 Add the `<mat-slider>` tag to the `app.component.html` like so:
 
 ```html
-<mat-slider min="1" max="100" step="1" value="1"></mat-slider>
+<mat-slider min="1" max="100" step="1" value="50"></mat-slider>
 ```
 
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