DOC: changed axes_api from automodule to autoclass

with automodule we had: `matplotlib.axes._axes.Axes`
while with autoclass we have `matplotlib.axes.Axes`

However, we must remember this documents only one class, so if
classes will be added to the module, they should be added
by hand.

Author: Yaron de Leeuw

doc/api/axes_api.rst

@@ -6,7 +6,7 @@ axes
 :mod:`matplotlib.axes`
 ======================
 
-.. automodule:: matplotlib.axes._axes
+.. autoclass:: matplotlib.axes.Axes
    :members:
    :undoc-members: