Update docs/blog/_posts/2021-02-17-scala3-rc1.md

Co-authored-by: Master-Killer <[email protected]>

Author: anatoliykmetyuk

docs/blog/_posts/2021-02-17-scala3-rc1.md

@@ -18,7 +18,7 @@ Type parameters on extensions can now be combined with type parameters on the me
 
 ```scala
 List(1, 2, 3).second[Int]
-extension [T](xs: List[T])
+extension [A](xs: List[A])
    def sumBy[B](f: A => B)(using Numeric[B]): B = ...
 ```