encoding/jsonschema: fix printing of error message

Change-Id: I19988edaa062f6fde0f1dcf988fb62395e827078
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6324
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/jsonschema/constraints.go b/encoding/jsonschema/constraints.go
index 49552a8..bc45cdf 100644
--- a/encoding/jsonschema/constraints.go
+++ b/encoding/jsonschema/constraints.go
@@ -444,7 +444,7 @@
 		obj := s.object(n)
 
 		if n.Kind() != cue.StructKind {
-			s.errf(n, `"properties" expected an object, found %v`, n.Kind)
+			s.errf(n, `"properties" expected an object, found %v`, n.Kind())
 		}
 
 		s.processMap(n, func(key string, n cue.Value) {