Fix default_url_options example in i18n guide

i18n guide has an example, which can break normal `link_to` workflow (somebody forgot about options).

Also, links in these quotes are broken, and I can't understand what was their target:
```
Rails contains infrastructure for "centralizing dynamic decisions about the URLs" in its [`ApplicationController#default_url_options`](http://api.rubyonrails.org/classes/ActionController/Base.html#M000515), which is useful precisely in this scenario: it enables us to set "defaults" for [`url_for`](http://api.rubyonrails.org/classes/ActionController/Base.html#M000503) and helper methods dependent on it (by implementing/overriding this method).
```