Fix a typo in an error message

Also remove parens because we ended up with
"... since the scrutinee type ((item: Int))" when the scrutinee is a
singleton type.

Author: smarter

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2126,7 +2126,7 @@ import transform.SymUtils._
       val addendum =
         if (scrutTp != testTp) s" is a subtype of ${testTp.show}"
         else " is the same as the tested type"
-      s"The highlighted type test will always succeed since the scrutinee type ($scrutTp.show)" + addendum
+      s"The highlighted type test will always succeed since the scrutinee type ${scrutTp.show}" + addendum
     }
     def explain = ""
   }