Bug in cancelTrip method from User datasource

https://github.com/apollographql/fullstack-tutorial/blob/66431a74abbdfd432157ec36a2fb7cd9f0b80be4/start/server/src/datasources/user.js#L60

Due to this line `cancelTrip` throws an error.

```sh
{
"errors": [
{
"message": "Cannot read property 'flight_number' of undefined",
"locations": [
{
"line": 2,
"column": 3
}
],
"path": [
"cancelTrip"
],
"extensions": {
"code": "INTERNAL_SERVER_ERROR",
"exception": {
"stacktrace": [
"TypeError: Cannot read property 'flight_number' of undefined",
// ....
```
Following change will fix the error.

```
async cancelTrip({ launchId }) {
const userId = this.context.user.id;
const numberOfDeletedTrips = await this.store.trips.destroy({ where: { userId, launchId } });
return numberOfDeletedTrips !== 0;
}
```
As per the [doc of `destroy`](https://sequelize.org/master/class/lib/model.js~Model.html#static-method-destroy) it returns number of deleted resources wrapped in Promise. So it should be resolved to get the count to decide if resources are deleted or not.