pkg/text/template: fix Execute builtin

Change-Id: I3fb79c347c2d0878ea0fdb062f53f9903bd9461a
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2041
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/pkg/text/template/manual.go b/pkg/text/template/manual.go
index 3895917..646f5e8 100644
--- a/pkg/text/template/manual.go
+++ b/pkg/text/template/manual.go
@@ -27,8 +27,12 @@
 	if err != nil {
 		return "", err
 	}
+	var x interface{}
+	if err := data.Decode(&x); err != nil {
+		return "", err
+	}
 	buf := &bytes.Buffer{}
-	if err := t.Execute(buf, data); err != nil {
+	if err := t.Execute(buf, x); err != nil {
 		return "", err
 	}
 	return buf.String(), nil