fix subscription check output and usage string

[hauksta]

Hi,

while using the plugin I noticed an error in the output for the subscription status when defining thresholds, e.g:

./check_pve.py -u <API_USER> -p <API_PASSWORD> -e <API_ENDPOINT> -m subscription -n node1 -w 50 -c 10

Once the license is about to expire the output is:

CRITICAL - {subscription_product_name} will expire in {delta} days ({subscription_due_date})

This PR fixes the output so it will print the correct subscription_product_name and subscription_due_date again.
Also noticed a minor issue in the usage output.

Thanks for making this very useful plugin !

[nbuchwitz]

I've re-formated your patch so the linter is happy. Otherwise LGTM, thanks!

[hauksta]

If the linter is happy, I am happy too. :-)
Thanks !