Swap order of PuOr scheme.

[jheer]

Fixes d3/d3-scale-chromatic#5.

Changes:

• Swap order of colors in PuOr scheme to match ColorBrewer.
• Update img/PuOr.png file for use in README.md

[mbostock]

Thanks for doing this. I think there might be a couple typos. I ran this quick test:

d3.select("body")
    .text(null)
  .selectAll(".scheme")
  .data(scheme)
  .enter().append("div")
    .attr("class", "scheme")
  .selectAll(".swatch")
  .data(scheme => scheme || [])
  .enter().append("div")
    .attr("class", "swatch")
    .style("display", "inline-block")
    .style("width", "20px")
    .style("height", "20px")
    .style("background-color", color => color);

Before:

After:

[mbostock]

Okay, fixed! Thank you.