fix document link on ngx_mruby gh_pages.

[d6rkaiz]

fix document link on ngx_mruby gh_pages.

Pull-Request Check List

• Add patches into src/.
• Add test into test/. Please see about test docs.
• Add docs into docs/ if you change the features such as build system, Ruby methods, class and nginx directives.

[matsumotory]

Thanks!