Use the #id reader instead of the nil @id variable

When template ID argument finally made it to url encoding when `String#gsub` was called on `@id` the variable would be nil. This change uses the id getter method instead of the absent `@id` instance variable (`ScanTemplate` never sets it while `ScanTemplateSummary` does).
No, your fix is what it should do. Surprised this wasn't noticed earlier, but guess people were using the other method.