tool_paramhlp: Fixed display of URL index in password prompt for --next

Commit f3bae6ed added the URL index to the password prompt when using
--next. Unfortunately, because the size_t specifier (%zu) is not
supported by all sprintf() implementations we use the curl_off_t format
specifier instead. The display of an incorrect value arises on platforms
where size_t and curl_off_t are of a different size.