README: add missing quotes

Author: matthiez

README.md

@@ -22,7 +22,7 @@ use Sms77\Api\Params\SmsParams;
 $client = new Client('MY_VERY_SECRET_API_KEY!');
 $params = new SmsParams();
 $client->sms($params
-    ->setTo(+4901234567890)
+    ->setTo('+4901234567890')
     ->setText('HI2U'));
 ```
 
@@ -164,4 +164,4 @@ particular API key. SMS77_RECIPIENT is the recipient of the test SMS.
 
 Need help? Feel free to [contact us](https://www.sms77.io/en/company/contact/).
 
-[![MIT](https://img.shields.io/badge/License-MIT-teal.svg)](./LICENSE)
+[![MIT](https://img.shields.io/badge/License-MIT-teal.svg)](./LICENSE)