250 hemispheric plot example updates

Need to correct this comment and potentially work-around an issue with TDS5 and the data request below only getting one single longitude (unidata/tds#315):
```python
# Set the lat/lon box for the data you want to pull in.
# lonlat_box(north_lat,south_lat,east_lon,west_lon)
gfsdata_hght.lonlat_box(0, 360, 0, 90)
```