DOC: Fix instructions for testing xarray
DOC: Fix instructions for testing xarray

chunweiyuan/xarray,pydata/xarray,xray/xray,shoyer/xray,pydata/xarray,markelg/xray,markelg/xray,rabernat/xray,shoyer/xarray,markelg/xray,jhamman/xarray,jcmgray/xarray,jhamman/xray,pydata/xarray,rabernat/xray,jhamman/xarray,shoyer/xarray,jhamman/xarray