Update a section that was using deprecated code

[ghost]

A bit of example code on https://square.github.io/intro-to-d3/data-binding/ was still using the deprecated rangeRoundBands method. The whole section has been updated to use a band scale instead of an ordinal one.

I am not sure the description is correct enough after being updated though.

I got some errors when I ran the code while following along your useful guide, so I figured this would be great for one of my first contributions to open source.

This would fix issue #22.

[jnothman]

I made similar changes to my copy of the code when doing the tutorial. +1