fix(ratio): missing variable prefix (#32501)

This is the only unprefixed custom property, and its name is very common so I think we'd better prefix it too.

site/assets/scss/_component-examples.scss

@@ -187,7 +187,7 @@
     width: 16rem;
 
     @include media-breakpoint-up(md) {
-      --aspect-ratio: 50%; // 2x1
+      --bs-aspect-ratio: 50%; // 2x1
     }
   }
 }