docs: update scheduled backup docker deployment example

docs/index.md

@@ -93,7 +93,7 @@ networks:
  -e "DB_HOST=hostname" \
  -e "DB_USERNAME=user" \
  -e "DB_PASSWORD=password" \
- jkaninda/pg-bkup backup -d dbName --cron-expression "@every 1m"
+ jkaninda/pg-bkup backup -d dbName --cron-expression "@every 15m" #@midnight
 ```
 See: https://jkaninda.github.io/pg-bkup/reference/#predefined-schedules