Fix T7438 output

Signed-off-by: Austin Seipp <austin@well-typed.com>

testsuite/tests/polykinds/T7438.stderr

@@ -4,7 +4,7 @@ T7438.hs:6:14:
       ‘t’ is untouchable
         inside the constraints (t2 ~ t3)
         bound by a pattern with constructor
-                   Nil :: forall (a :: k). Thrist a a,
+                   Nil :: forall (k :: BOX) (a :: k). Thrist a a,
                  in an equation for ‘go’
         at T7438.hs:6:4-6
       ‘t’ is a rigid type variable bound by