Fixed first example in Chapter 5

The legend was not being shown properly for the 3 subplots in the first example. It looks like plt.fill_between() wont work with a normal legend() call. 

Thus, I tweaked the code a bit following the suggestion in this SO thread:
http://stackoverflow.com/questions/14534130/legend-not-showing-up-in-matplotlib-stacked-area-plot

Hope this makes sense!
Also, this is my first ever contribution on github. This is so cool!!!