Update api_key_authentication.rst - POST Method

Can be useful if your POST / PUT / DELETE api's methods needs a token
I would write the code like this:

```php
// look for an apikey query parameter
$apiKey = $request->query->get('apikey');

// or if you want to use an "apikey" header, then do something like this:
// $apiKey = $request->headers->get('apikey');

if (!$apiKey) {
    throw new BadCredentialsException('No API key found');
}

return new PreAuthenticatedToken(
    'anon.',
    $apiKey,
    $providerKey
);
```

@BourotBenjamin What do you think about this?