part12b - highlight changed line

While syntactic difference might be small, semantically it's pretty significant, and it's easy to miss this change, so I think it's important to highlight it.

Author: watteja

src/content/12/en/part12b.md

@@ -560,7 +560,7 @@ services:
       MONGO_INITDB_DATABASE: the_database
     volumes:
       - ./mongo/mongo-init.js:/docker-entrypoint-initdb.d/mongo-init.js
-      - mongo_data:/data/db
+      - mongo_data:/data/db # highlight-line
 
 volumes: # highlight-line
   mongo_data: # highlight-line