add missing '.html' in doc/reference_manual/index.rst
@@ -87,4 +87,4 @@ HoloViews subpackages
.. _plotting.plotly: holoviews.plotting.plotly.html
.. _selection: holoviews.selection.html
.. _streams: holoviews.streams.html
-.. _util: holoviews.util
+.. _util: holoviews.util.html
