Fix go vet issue

decode.go

diff --git a/decode.go b/decode.go
index b989e63..b717a64 100644
--- a/decode.go
+++ b/decode.go
@@ -190,7 +190,7 @@
 			fv := v.Field(i)
 			fk, def, opts := keydef(t.Field(i))
 			if !fv.CanSet() {
-				return fmt.Errorf("cannot set ", t.Field(i).Name)
+				return fmt.Errorf("cannot set %s", t.Field(i).Name)
 			}
 			if fk == "-" {
 				continue