Update README.md

Fixed function signature of repo.contents()

Author: jacobrossi

README.md

@@ -38,10 +38,10 @@ Show repository information
 repo.show(function(err, repo) {});
 ```
 
-Get contents at a particular path.
+Get contents at a particular path in a particular branch.
 
 ```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.