doc: Change qvm-service name from "cron" to "crond"

doc/qvm-tools/qvm-service.rst

@@ -73,7 +73,7 @@ cups
 
     Enable CUPS service. The user can disable cups in VM which do not need printing to speed up booting.
 
-cron
+crond
     Default: disabled
 
     Enable CRON service.