Update image source for kafbat

Kafbath publishes its images to Docker Hub, so changing the registry source to Hub instead of GHCR. Also, instead of using the latest tag, they publish merged changes onto a `main` tag.

Author: mikesir87

content/guides/kafka.md

@@ -235,7 +235,7 @@ To add it to your own project (it’s already in the demo application), you only
 ```yaml
 services:
   kafka-ui:
-    image: ghcr.io/kafbat/kafka-ui:latest
+    image: kafbat/kafka-ui:main
     ports:
       - 8080:8080
     environment: