DOC: Switch nested pie example from cmaps to color_sequences

This makes it clearer that we are just indexing into a list of colors. Calling cmaps with an integer argument is somewhat obscure and we want to move away from this. See also matplotlib#28198.

galleries/examples/pie_and_polar_charts/nested_pie.py

@@ -31,9 +31,9 @@
 size = 0.3
 vals = np.array([[60., 32.], [37., 40.], [29., 10.]])
 
-cmap = plt.colormaps["tab20c"]
-outer_colors = cmap(np.arange(3)*4)
-inner_colors = cmap([1, 2, 5, 6, 9, 10])
+tab20c = plt.color_sequences["tab20c"]
+outer_colors = [tab20c[i] for i in [0, 4, 8]]
+inner_colors = [tab20c[i] for i in [1, 2, 5, 6, 9, 10]]
 
 ax.pie(vals.sum(axis=1), radius=1, colors=outer_colors,
        wedgeprops=dict(width=size, edgecolor='w'))