Correct singlediode.bishop88_mpp docs (#1615)

* fix return docstring

* whatsnew

Author: cwhanse

docs/sphinx/source/whatsnew/v0.9.4.rst

@@ -65,6 +65,7 @@ Documentation
 ~~~~~~~~~~~~~
 * Added an FAQ page to the docs: :ref:`faq`. (:issue:`1546`, :pull:`1549`)
 * Fixed equation in :py:func:`pvlib.iam.martin_ruiz` docstring (:issue:`1561`, :pull:`1599`)
+* Fixed an error in :py:func:`pvlib.singlediode.bishop88_mpp` docstring (:issue:`1613`, :pull:`1615`)
 
 Benchmarking
 ~~~~~~~~~~~~~

pvlib/singlediode.py

@@ -422,7 +422,7 @@ def bishop88_mpp(photocurrent, saturation_current, resistance_series,
 
     Returns
     -------
-    OrderedDict or pandas.DataFrame
+    tuple
         max power current ``i_mp`` [A], max power voltage ``v_mp`` [V], and
         max power ``p_mp`` [W]
     """