Docs: Use absolute qualified name to generate links

This is more robust than letting sphinx look for FlaskUtils or init_app and create a link to the first thing it finds

docs/source/api.rst

@@ -17,7 +17,7 @@ Custom exceptions
 
 .. warning:: For any of these errors to work, you need to register the error handlers in your Flask app.
 
-    To do this, you need to pass :attr:`register_error_handlers=True` to the :class:`FlaskUtils` class or to :meth:`~FlaskUtils.init_app()`.
+    To do this, you need to pass :attr:`register_error_handlers=True` to the :class:`~flask_utils.extension.FlaskUtils` class or to :meth:`~flask_utils.extension.FlaskUtils.init_app()`.
 
     .. code-block:: python