chore(button-toggle): use more accurate example in docs (#10579)

Author: crisbeto

src/material-examples/button-toggle-overview/button-toggle-overview-example.html

@@ -1 +1,5 @@
-<mat-button-toggle>Toggle me!</mat-button-toggle>
+<mat-button-toggle-group name="fontStyle">
+  <mat-button-toggle value="bold">Bold</mat-button-toggle>
+  <mat-button-toggle value="italic">Italic</mat-button-toggle>
+  <mat-button-toggle value="underline">Underline</mat-button-toggle>
+</mat-button-toggle-group>