Named colors example, figure size correction [backport to 1.4.0-doc]

Looking at the gallery page, the named color example figure doesn't have the right size
http://matplotlib.org/gallery.html#color

I have corrected the example using only one axes, and referencing all the sizes to the actual figure size in points.
![figure_1](https://cloud.githubusercontent.com/assets/1490153/4255055/4979467a-3ab1-11e4-8de6-858a8ea7cf08.png)