Clarify what are "regular methods" in the extension methods section

Author: anatoliykmetyuk

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

@@ -28,7 +28,7 @@ Type arguments matching method type parameters are passed as usual:
 List("a", "bb", "ccc").sumBy[Int](_.length)
 ```
 By contrast, type arguments matching type parameters following `extension` can be passed
-only if the method is referenced as a regular method:
+only if the method is referenced as a non-extension method:
 
 ```scala
 sumBy[String](List("a", "bb", "ccc"))(_.length)