Fix resample docstring seasonal average example (#2268)
Fix resample docstring seasonal average example (#2268)

Closes #2232
pydata/xarray,jhamman/xarray,jhamman/xray,markelg/xray,shoyer/xarray,xray/xray,chunweiyuan/xarray,markelg/xray,jhamman/xarray,jhamman/xarray,markelg/xray,pydata/xarray,pydata/xarray,shoyer/xarray,shoyer/xray