Resource configuration is in wrong case

## Description

Using YAML or XML, the global configuration is in the following case:
```yaml
api_platform:
defaults:
pagination_client_enabled: true
```

But any resource configuration is in the following case:
```yaml
resources:
App\Entity\Book:
paginationClientEnabled: true
```
```xml

```

## Proposition

Following Symfony Best Practices, the configuration should be as following:
```yaml
resources:
App\Entity\Book:
pagination_client_enabled: true
```
```xml

```