Fix Ixx equation in pvsystem.sapm (pvlib#2019)

* fix Ixx equation, use Aimp

* Update docs/sphinx/source/whatsnew/v0.10.5.rst

* lint

* lint

* get the spacing right

* more spacing

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Co-authored-by: Kevin Anderson <[email protected]>

Author: 2 people

docs/sphinx/source/whatsnew/v0.10.5.rst

@@ -31,6 +31,7 @@ Bug fixes
 * Fixed :py:func:`pvlib.pvsystem.retrieve_sam` silently ignoring the ``path`` parameter
   when ``name`` was provided. Now an exception is raised requesting to only provide one
   of the two parameters. (:issue:`2018`, :pull:`2020`)
+* Corrected equation for Ixx0 in :py:func:`pvlib.pvsystem.sapm` (:issue:`2016`, :pull:`2019`)
 * Fixed :py:func:`pvlib.pvsystem.retrieve_sam` silently ignoring the `path` parameter
   when `name` was provided. Now an exception is raised requesting to only provide one
   of the two parameters. (:issue:`2018`, :pull:`2020`)