Polymorphic Relation - Wrong Doc

### Describe the bug

Hi there!

Let's suppose a base Delivery model with 3 fields: id, deliverableType, and deliverableId.
If you follow the [docs](https://loopback.io/doc/en/lb4/Polymorphic-relation.html), you'll get this error when requesting some deliverable relations:

> Request GET /deliveries?filter=whatever failed with status code 500. Error: ER_BAD_FIELD_ERROR: Unknown column 'deliverable' in 'field list'

This is caused by this part of code, as you're asking it to search on the corresponding datasource for the field "deliverable" which doesn't exist:

```
class Delivery extends Entity {
@hasOne(() => Deliverable, {polymorphic: true})
deliverable: Deliverable;

deliverableType: string;
}
```

Instead, it should be like this (search deliverableId on deliverableType's model):
```
class Delivery extends Entity {
@hasOne(() => Deliverable, {polymorphic: true})
deliverableId: string;

deliverableType: string;
id: string;
}
```

With that change, polymorphic relation is working as expected (assuming repositories are correctly configured)

### Logs

_No response_

### Additional information

_No response_

### Reproduction

Not needed

Start with the Polymorphic relation documentation at https://loopback.io/doc/en/lb4/Polymorphic-relation.html and compare its Delivery example with the issue's reported fields. Update the example so the relation uses deliverableId and deliverableType, then verify that the documented example no longer requests the nonexistent deliverable field.

Written by the indexing model from the issue text.