Prefer colorbar(ScalarMappable(...)) to ColorbarBase in tutorial.

Nearly all the colorbar_only tutorial already uses
`colorbar(ScalarMappable(...))` to generate "standalone" colorbars;
change the last example that was still using `ColorbarBase`.

tutorials/colors/colorbar_only.py

@@ -52,11 +52,10 @@
 cmap = mpl.cm.viridis
 bounds = [-1, 2, 5, 7, 12, 15]
 norm = mpl.colors.BoundaryNorm(bounds, cmap.N, extend='both')
-cb2 = mpl.colorbar.ColorbarBase(ax, cmap=cmap,
-                                norm=norm,
-                                orientation='horizontal')
-cb2.set_label("Discrete intervals with extend='both' keyword")
-fig.show()
+
+fig.colorbar(mpl.cm.ScalarMappable(norm=norm, cmap=cmap),
+             cax=ax, orientation='horizontal',
+             label="Discrete intervals with extend='both' keyword")
 
 ###############################################################################
 # Discrete intervals colorbar