present: fix typo

Fixes golang/go#18670.

Change-Id: Ia4e0d6b1d8fd78a165e31e72c17f520850da7a3f
Reviewed-on: https://go-review.googlesource.com/35258
Reviewed-by: Minux Ma <minux@golang.org>

present/iframe.go

@@ -39,7 +39,7 @@ func parseIframe(ctx *Context, fileName string, lineno int, text string) (Elem,
 			i.Width = v
 		}
 	default:
-		return nil, fmt.Errorf("incorrect image invocation: %q", text)
+		return nil, fmt.Errorf("incorrect iframe invocation: %q", text)
 	}
 	return i, nil
 }