Fix typo in readme for orders endpoint

[joshuatf]

This PR fixes the typo noted in #347 for the orders/ endpoint.

Testing

1. Verify that the updated endpoint matches that of the docs - https://woocommerce.github.io/woocommerce-rest-api-docs/#retrieve-an-order
2. Optionally test with the following snippet:

$woocommerce = new Client(
    'http://merchanturl.local',
    'consumer_key',
    'consumer_secret',
    [
        'wp_api'  => true,
        'version' => 'wc/v3',
    ]
);
$orders = $woocommerce->get('orders', ['status'=>'processing', 'per_page'=>1]);
print_r($orders);

[prettyboymp]

👍