Update spectral_factor_sapm docstring returns statement (#2122)

* update returns statement

* Update pvlib/spectrum/mismatch.py

Co-authored-by: Adam R. Jensen <39184289+AdamRJensen@users.noreply.github.com>

---------

Co-authored-by: Adam R. Jensen <39184289+AdamRJensen@users.noreply.github.com>

Author: RDaxini

pvlib/spectrum/mismatch.py

@@ -559,7 +559,7 @@ def spectral_factor_sapm(airmass_absolute, module):
     Returns
     -------
     f1 : numeric
-        The SAPM spectral loss coefficient.
+        The spectral mismatch factor. [unitless]
 
     Notes
     -----