Fix `chapter_links` to be shown at '/guides/enumerables'

`chapter_links` wasn't rendered at `/guides/enumerables/`.
This cause is:
This section name is 'enumerables'.
But `chapter_links` would match it with 'enumerables/' (end with `/`).

So I fixed to match url to section slug.

before:
![2013-05-05 18 29 56](https://f.cloud.github.com/assets/290782/463481/6c26f418-b567-11e2-942c-57b925181242.png)

After:
![2013-05-05 18 30 08](https://f.cloud.github.com/assets/290782/463482/70c0632e-b567-11e2-94a4-2a35aea1f1c9.png)