fix spectral model typo (#255)

Author: wholmgren

docs/sphinx/source/whatsnew/v0.4.2.txt

@@ -12,6 +12,8 @@ Bug fixes
 * Fixed typo in __repr__ method of ModelChain and in its regarding test. (commit: b691358b)
 * PVSystem.pvwatts_ac could not use the eta_inv_ref kwarg and
   PVSystem.pvwatts_dc could not use the temp_ref kwarg. Fixed. (:issue:`252`)
+* Fixed typo in ModelChain.infer_spectral_model error message. (:issue:`251`)
+
 
 API Changes
 ~~~~~~~~~~~

pvlib/modelchain.py

@@ -528,7 +528,7 @@ def infer_spectral_model(self):
         if set(['A4', 'A3', 'A2', 'A1', 'A0']) <= params:
             return self.sapm_spectral_loss
         else:
-            raise ValueError('could not infer temp model from ' +
+            raise ValueError('could not infer spectral model from ' +
                              'system.module_parameters')
 
     def first_solar_spectral_loss(self):