Privacy policy: update "Credits" and relicense under CC BY-SA 4.0

The attribution reads much better when the license of this document is
pointed out explicitly, and why should we not use the latest version?

privacy-policy.md

@@ -143,4 +143,4 @@ This is version 1.0. Last updated on 10 March, 2019.
 
 ## Credits
 
-This document is based on the [Fedora Project's Privacy Policy](https://fedoraproject.org/wiki/Legal:PrivacyPolicy) available under the terms of the [Creative Commons Attribution-Share Alike 3.0 Unported license](https://creativecommons.org/licenses/by-sa/3.0/).
+This document is based on the [Fedora Project's Privacy Policy](https://fedoraproject.org/wiki/Legal:PrivacyPolicy), used under the terms of the [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0/) license. This document is licensed under the terms of [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/).