Update README.md

Documented branch parameter for ´contents()´

Author: Jan Marsch

README.md

@@ -54,7 +54,7 @@ repo.show(function(err, repo) {});
 Get contents at a particular path.
 
 ```js
-repo.contents("path/to/dir", function(err, contents) {});
+repo.contents("master", "path/to/dir", function(err, contents) {});
 ```
 
 Fork repository. This operation runs asynchronously. You may want to poll for `repo.contents` until the forked repo is ready.