[ci skip] Update guides license to CC BY-SA 4.0 International.

Benefits: https://creativecommons.org/version4

guides/source/_license.html.erb

-<p>This work is licensed under a <a href="http://creativecommons.org/licenses/by-sa/3.0/">Creative Commons Attribution-Share Alike 3.0</a> License</p>
+<p>This work is licensed under a <a href="https://creativecommons.org/licenses/by-sa/4.0/">Creative Commons Attribution-ShareAlike 4.0 International</a> License</p>
 <p>"Rails", "Ruby on Rails", and the Rails logo are trademarks of David Heinemeier Hansson. All rights reserved.</p>