[doc] fix code-block issue with extra line

doc/index.rst

@@ -51,6 +51,7 @@ Here are the `documentation <https://gitlab.idiap.ch/beat/beat.backend.python/bl
 The ``index()`` function:
 
 .. code-block:: python
+
     def index(self, root_folder, parameters):
         """Returns a list of (named) tuples describing the data provided by the view.