Fix default_url_options example in i18n guide

[killthekitten]

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).

[rafaelfranca]

About the broken links, the first link should point to http://api.rubyonrails.org/classes/ActionDispatch/Routing/Mapper/Base.html#method-i-default_url_options and the later to http://api.rubyonrails.org/classes/ActionDispatch/Routing/UrlFor.html#method-i-url_for

Mind to update the PR?

[killthekitten]

@rafaelfranca 👍 thank you for help, didn't have time to find this links by myself

[killthekitten]

@rafaelfranca and I'm not sure is it OK to reverse merge options like I did in example?

I mean, should we keep this:

{ locale: I18n.locale }.merge options

or this:

options.merge{ locale: I18n.locale }

[rafaelfranca]

The first is better. If you want to override in the call of url_for it should be possible.

So with first url_for(:locale => :us) would work. With the second it will always use the I18n.locale

[rafaelfranca]

Thank you for the patch