Update README.md

Co-authored-by: Christian Lück <[email protected]>

Author: 51imyyy

README.md

@@ -2391,12 +2391,7 @@ add a request header for all following requests.
 ```php
 $browser = $browser->withHeader('User-Agent', 'ACME');
 
-$browser->get($url)->then(function (Psr\Http\Message\ResponseInterface $response) {
-    //will contain the header User-Agent with the value ACME
-    var_dump($response);
-}, function (Exception $e) {
-    echo 'Error: ' . $e->getMessage() . PHP_EOL;
-});
+$browser->get($url)->then(…);
 ```
 
 Note that the new header will overwrite any headers previously set with