Update 2019-12-20-21th-dotty-milestone-release.md

reset the extension syntax to the version included in the release

Author: bishabosha

docs/blog/_posts/2019-12-20-21th-dotty-milestone-release.md

@@ -183,7 +183,7 @@ given extension (s: String) { ... }
 ```
 or
 ```scala
-extension listOps of [T](xs: List[T]) with { ... }
+given listOps: [T](xs: List[T]) extended with { ... }
 
 given (s: String) extended with { ... }
 ```