Minor improvements to "PV System Models" reference page (#2014)

list spectrum.spectral_factor_sapm, correct PVsyst capitalization

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

@@ -13,10 +13,11 @@ Sandia array performance model (SAPM)
    pvsystem.sapm
    pvsystem.sapm_effective_irradiance
    pvsystem.sapm_spectral_loss
+   spectrum.spectral_factor_sapm
    inverter.sandia
    temperature.sapm_cell
 
-Pvsyst model
+PVsyst model
 ^^^^^^^^^^^^
 
 .. autosummary::