Fix amazon affiliate ID link

I found that generated URLs with amazon affiliate ID may be invalid.

For example:

* Invalid: http://amazon.co.jp/dp/4832244140/your-affiliate-22
* Valid: http://amazon.co.jp/dp/4832244140/?tag=your-affiliate-22

You can check it in below URL.
https://affiliate.amazon.co.jp/gp/associates/network/tools/link-checker/main.html

So this pull-req fixes this problem.