Update forecast.py comment typo

Fix minor typo in comment from DHI to GHI

Author: aperetti

pvlib/forecast.py

@@ -441,7 +441,7 @@ def cloud_cover_to_irradiance_clearsky_scaling(self, cloud_cover,
            cloud_cover e.g.
            :py:meth:`~ForecastModel.cloud_cover_to_ghi_linear`
         3. Estimate cloudy sky DNI using the DISC model.
-        4. Calculate DHI from DNI and DHI.
+        4. Calculate DHI from DNI and GHI.
 
         Parameters
         ----------