builder/amazon/ebs: fix incorrect printf verb type

Fixes the following vet report:

builder/amazon/ebs/step_create_ami.go:90: arg resp.Return for printf verb %s of wrong type: bool

builder/amazon/ebs/step_create_ami.go

@@ -87,7 +87,7 @@ func (s *stepCreateAMI) Cleanup(state multistep.StateBag) {
 		ui.Error(fmt.Sprintf("Error deregistering AMI, may still be around: %s", err))
 		return
 	} else if resp.Return == false {
-		ui.Error(fmt.Sprintf("Error deregistering AMI, may still be around: %s", resp.Return))
+		ui.Error(fmt.Sprintf("Error deregistering AMI, may still be around: %t", resp.Return))
 		return
 	}
 }