adopt observable10

docs/lib/d3.md

@@ -11,7 +11,7 @@ For example, here is an interactive [force-directed](https://github.com/d3/d3-fo
 ```js echo
 const width = 640;
 const height = 640;
-const color = d3.scaleOrdinal(d3.schemeCategory10);
+const color = d3.scaleOrdinal(d3.schemeObservable10);
 
 // Copy the data to protect against mutation by d3.forceSimulation.
 const links = data.links.map((d) => Object.create(d));