Adjust failing expectation in WUnused tests

Author: szymon-rd

tests/neg-custom-args/fatal-warnings/i15503f.scala

@@ -8,7 +8,7 @@ object Xd {
   private def f2(a: Int) = 1 // OK
   private def f3(a: Int)(using Int) = a // OK
   private def f4(a: Int)(using Int) = default_int // OK
-  private def f6(a: Int)(using Int) = summon[Int] // error
+  private def f6(a: Int)(using Int) = summon[Int] // OK
   private def f7(a: Int)(using Int) = summon[Int] + a // OK
   private def f8(a: Int)(using foo: Int) = a // error
 }