No example for renaming subscription operations and the significance of the different format

**Describe the content issue**:
It looks like the structure for how to define custom operation names for `subscriptions` is different than `mutations` and `queries`, as the docs highlight:

```graphql
input ModelMutationMap {
create: String
update: String
delete: String
}

input ModelQueryMap {
get: String
list: String
}

input ModelSubscriptionMap {
onCreate: [String]
onUpdate: [String]
onDelete: [String]
level: ModelSubscriptionLevel
}
```

The `subscriptions` take an array of strings while the others take standalone strings, but why it's different and how to leverage it is unclear. Should it be a single string inside the array, making it something like this by default `onUpdate: "onUpdate{TypeName}"`? Are more names per operation supported? Or is it just a quirk of how GraphQL directives must be made that it has to be an array and we should only include a single entry?

**URL page where content issue is**:
https://docs.amplify.aws/react/build-a-backend/graphqlapi/data-modeling/