fix examples for revealjs image background

Author: cderv

docs/presentations/revealjs/examples/image-background.qmd

@@ -2,6 +2,6 @@
 format: revealjs
 ---
 
-## Slide Title {background-color="black" background-image="https://placekitten.com/100/100" background-size="100px" background-repeat="repeat"}
+## Slide Title {background-color="black" background-image='{{< placeholder 100 100 >}}' background-size="100px" background-repeat="repeat"}
 
 This slide's background image will be sized to 100px and repeated.