documentation: cron examples: replace shell by btrbk

There's no point of having the shell process around while the backup is
running.

README.md

@@ -147,7 +147,7 @@ Retention policy:
 /etc/cron.daily/btrbk:
 
     #!/bin/sh
-    /usr/sbin/btrbk -c /etc/btrbk/btrbk-mylaptop.conf run
+    exec /usr/sbin/btrbk -c /etc/btrbk/btrbk-mylaptop.conf run
 
 
 - This will create snapshots on a daily basis:
@@ -227,7 +227,7 @@ regular basis:
 /etc/cron.daily/btrbk:
 
     #!/bin/sh
-    /usr/sbin/btrbk run
+    exec /usr/sbin/btrbk run
 
 Note that you can run btrbk more than once a day, e.g. by creating the
 above script in `/etc/cron.hourly/btrbk`, or by calling `sudo btrbk