Update laravel/documentation/localization.md

Updated to use the two-letter, ISO standard language code for Spanish--'es' instead of 'sp'.

Just a minor change. But I think it's important to use the ISO standard lang code in the docs in order to encourage other developers to stick to this standard convention too for consistency across bundles and various lang files.

ISO language codes: http://www.loc.gov/standards/iso639-2/php/code_list.php