fix minor docs error from #2244

Author: kandersolar

docs/sphinx/source/reference/pv_modeling/system_models.rst

@@ -12,7 +12,7 @@ Sandia array performance model (SAPM)
 
    pvsystem.sapm
    pvsystem.sapm_effective_irradiance
-   pvsystem.sapm_spectral_loss
+   spectrum.spectral_factor_sapm
    inverter.sandia
    temperature.sapm_cell