remove docblock about multiple requests handling

With php-http#75 being merged the `HttpClient` interface no longer supports
sending multiple requests at once.

src/HttpClient.php

@@ -7,7 +7,7 @@
 use Psr\Http\Message\ResponseInterface;
 
 /**
- * Sends one or more PSR-7 Request and returns PSR-7 responses.
+ * Sends a PSR-7 Request and returns a PSR-7 response.
  *
  * @author GeLo <[email protected]>
  * @author Márk Sági-Kazár <[email protected]>