Fix documentation link
Fix documentation link

Fixes #756

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