…ayloadcms#16183)

### What?

Fix incorrect property name in documentation for defining a Collection
Config.

### Why?

The documentation incorrectly referenced `collection` instead of the
correct `collections` property in the Payload Config. This could mislead
users and cause configuration errors.

### How?

Updated the documentation to replace `collection` with `collections` in
the relevant section.

### Before

```ts
collection: [...]
```

### After

```ts
collections: [...]
```

Fixes # (N/A)