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/'.

So I fixed to match url to section slug.
Merge pull request #479 from tricknotes/fix-chapter-links

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