Added docs for constructor argument

Author: Nyholm

doc/api_version.md

@@ -1,14 +1,13 @@
 ## Api version
 [Back to the navigation](README.md)
 
-If you want to change the API version from its default ("v3") you may do that with
-the `setApiVersion` function. 
+If you want to change the API version from its default ("v3") you may do that with a constructor argument.
 For example:
 
 ```php
 $client = new Github\Client();
-
 echo $client->getApiVersion(); // prints "s3"
 
-$client->setApiVersion("v2");
+$client = new Github\Client($httpClient, 'v2');
+echo $client->getApiVersion(); // prints "s2"
 ```