Terms of Service: add link to the license in attribution

Adding a link is nice (and maybe even required by the license), using
abbreviations is acceptable as per the examples here:
https://wiki.creativecommons.org/wiki/License_Versions#Attribution-specific_elements

terms-of-service.md

@@ -166,4 +166,4 @@ About This Document
 -------------------
 
 This document was derived from the Automattic Terms of Service and is licensed
-under a Creative Commons Sharealike license (CC BY-SA 4.0).
+under [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/).