Correct the documentation for connection configuration.

The documentation currently specifies:

```php
<?php
$es = Client::connection(array(
    'server' => '127.0.0.1:9200',
    'protocol' => 'http',
    'index' => 'myindex',
    'type' => 'mytype'
));
```

But the correct config key according to the code is `servers`. Might save someone else a few minutes of grokking :)