Merge pull request #8132 from taisukeoe/patch-2

fix union type sample code to compile

Author: nicolasstucki

docs/docs/reference/new-types/union-types-spec.md

@@ -71,6 +71,7 @@ Given
 ```scala
 trait C[+T]
 trait D
+trait E
 class A extends C[A] with D
 class B extends C[B] with D with E
 ```