Update installing.rst
Update installing.rst

Corrected poor formatting and clarify new conda-forge text.
jhamman/xarray,shoyer/xarray,pydata/xarray,jcmgray/xarray,jhamman/xarray,xray/xray,jhamman/xray,markelg/xray,markelg/xray,pydata/xarray,jhamman/xarray,markelg/xray,rabernat/xray,pydata/xarray,rabernat/xray,shoyer/xarray,shoyer/xray,chunweiyuan/xarray