[skip ci] Fixed rest client instantiation in documentation

Author: antonkomarev

README.md

@@ -117,9 +117,11 @@ YOUTRACK_PASSWORD=secret
 ### Initialize API client
 
 ```php
-$youtrack = app(\Cog\YouTrack\Rest\YouTrackClient::class);
+$youtrack = app(\Cog\Contracts\YouTrack\Rest\Client\Client::class);
 ```
 
+As result instantiated `\Cog\YouTrack\Rest\Client\YouTrackClient` class should be returned.
+
 ### API requests
 
 #### HTTP request