Adjust cron expression and config comments.

foundation-core/src/main/kotlin/cloud/kubelet/foundation/core/features/backup/BackupFeature.kt

@@ -29,7 +29,9 @@ class BackupFeature : Feature() {
     registerCommandExecutor("fbackup", BackupCommand(plugin, backupPath, config, s3Client))
 
     if (config.schedule.cron.isNotEmpty()) {
-      scheduleId = scheduler.cron("${config.schedule.cron} ?") {
+      // Assume user never wants to modify second. I'm not sure why this is enforced in Quartz.
+      val expr = "0 ${config.schedule.cron}"
+      scheduleId = scheduler.cron(expr) {
         plugin.server.scheduler.runTask(plugin) { ->
           plugin.server.dispatchCommand(plugin.server.consoleSender, "fbackup")
         }

foundation-core/src/main/resources/backup.yaml

@@ -1,8 +1,9 @@
 # Configuration of backup scheduling, expressed by cron expressions.
 schedule:
-  # Cron expression to use for the backup schedule.
+  # Cron expression to use for the backup schedule. This is not standard cron, but rather a subset
+  # that is given to us by the Quartz Scheduler.
   # Examples:
-  # "0 3 * * *"   -> every day at 3 AM
+  # "0 3 * * ?"   -> every day at 3 AM
   # "0 3 * * SUN" -> every Sunday at 3 AM
   cron: ""