D3 example tighten

I was reading the "rendering data as graphs" article and wanted to fix some small consistency issues (OctoKit versus Octokit, and using "D3", more canonical than the lowercase "d3") and a couple of typos.

I ended up making some prose updates that I felt tightened up or otherwise improved the flow of the guide, but I can submit a different pull with just spelling corrections/consistency if you're not into the changes. Just let me know.