docs(backup): default value for completionWindow is incorrect (#27061)

Currently, the description regarding default value for `completionWindow` is incorrect.
https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_backup.BackupPlanRuleProps.html#completionwindow
> Type: Duration (optional, default: 8 hours)

Correct value is 7 days (10080 minutes).

Closes #27050

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packages/aws-cdk-lib/aws-backup/lib/rule.ts

@@ -10,7 +10,7 @@ export interface BackupPlanRuleProps {
    * The duration after a backup job is successfully started before it must be
    * completed or it is canceled by AWS Backup.
    *
-   * @default - 8 hours
+   * @default - 7 days
    */
   readonly completionWindow?: Duration;