Allow to search for 08 (Chapter 8).

Currently search will fail when searching for '08' and '09' (Chapter 8 and 9). I'm not entirely sure why, but:

```ruby
~ (main) > sprintf("%02d", "7")
=> "07"
~ (main) > sprintf("%02d", "8")
=> "08"
~ (main) > sprintf("%02d", "07")
=> "07"
~ (main) > sprintf("%02d", "08")
ArgumentError: invalid value for Integer(): "08"
from (pry):4:in `sprintf'
~ (main) > sprintf("%02d", "09")
ArgumentError: invalid value for Integer(): "09"
from (pry):5:in `sprintf'
~ (main) > sprintf("%02d", "10")
=> "10"
```

So this change fixes it.