DOC: update very old whats new entry to new usage

Author: tacaswell

examples/pyplots/whats_new_99_mplot3d.py

@@ -17,7 +17,8 @@
 Z = np.sin(R)
 
 fig = plt.figure()
-ax = Axes3D(fig)
+ax = Axes3D(fig, auto_add_to_figure=False)
+fig.add_axes(ax)
 ax.plot_surface(X, Y, Z, rstride=1, cstride=1, cmap=cm.viridis)
 
 plt.show()