Fix contributing link

Author: dstansby

docs/installation.rst

@@ -34,4 +34,4 @@ Zarr has endorsed `Scientific-Python SPEC 0 <https://scientific-python.org/specs
 
 Development
 -----------
-To install the latest development version of Zarr, see :ref:`contributing`.
+To install the latest development version of Zarr, see `the contributing guide <contributing.html>`_.