docs: fix widget example

Author: andrewseguin

guides/theming-your-components.md

@@ -781,7 +781,7 @@ Taking it one step further, you can alternatively use the utility classes to ach
 in the component template:
 
 ```html
-<widget-a class="mat-bg-surface mat-font-body-medium"/>
+<widget-a class="mat-bg-surface mat-text-on-surface mat-font-body-medium"/>
 
-<widget-b class="mat-bg-primary"/>
+<widget-b class="mat-bg-primary mat-text-on-primary"/>
 ```