plot_adaboost_twoclass.py: minor improvements

Before:

![figure_1a](https://cloud.githubusercontent.com/assets/202816/4486532/c0900c02-49e4-11e4-8b70-35193f04ae63.png)

After:

![figure_1](https://cloud.githubusercontent.com/assets/202816/4486533/c546513e-49e4-11e4-849c-ddeca326bc61.png)