url_handlers: Add parenthesis around the MAX_RECEIPT_AGE sum
url_handlers: Add parenthesis around the MAX_RECEIPT_AGE sum

This was recommended by checkpatch.pl

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@opentechlabs.co.uk>

ac000/receiptomatic