Fix docs: Crontabs aren't executable (#1039).

Reviewed-on: https://projects.torsion.org/borgmatic-collective/borgmatic/pulls/1039

docs/how-to/set-up-backups.md

@@ -311,7 +311,6 @@ Then, from the directory where you downloaded it:
 
 ```bash
 sudo mv borgmatic /etc/cron.d/borgmatic
-sudo chmod +x /etc/cron.d/borgmatic
 ```
 
 If borgmatic is installed at a different location than