Update wording in SearchIndex get_model exception

Thanks to Greg Brown (@gregplaysguitar) for the patch

Closes django-haystack#1223

Author: acdha

haystack/indexes.py

@@ -124,7 +124,7 @@ def get_model(self):
 
         This method is required & you must override it to return the correct class.
         """
-        raise NotImplementedError("You must provide a 'model' method for the '%r' index." % self)
+        raise NotImplementedError("You must provide a 'get_model' method for the '%r' index." % self)
 
     def index_queryset(self, using=None):
         """