fix path for assets

[stakahashi]

https://ja.wp-api.org/reference/posts/

Asset link is broken.

fixed.
http://dev-wp-api-docs.azsx.de/reference/posts/

[naokomc]

@miya0001 - could you take a look at this?
Another user reported the issue that CSS is not applied to http://ja.wp-api.org/reference/ etc.

Thanks!

[ixkaito]

@stakahashi @naokomc @miya0001
Actually, {{ site.baseurl }} is set as / in _config.yml, so this PR isn't a real solution to the problem. I sent a new PR #19 to fix this issue.