Update testing instructions to use pytest

As `pytest` is now the way to test `zarr`, update the testing
instructions to use `pytest`.

Author: jakirkham

docs/index.rst

@@ -48,8 +48,8 @@ To work with Zarr source code in development, install from GitHub::
 
 To verify that Zarr has been fully installed, run the test suite::
 
-    $ pip install nose
-    $ python -m nose -v zarr
+    $ pip install pytest
+    $ python -m pytest -v --pyargs zarr
 
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