Update CONTRIBUTING

Author: mbostock

CONTRIBUTING.md

@@ -99,7 +99,7 @@ The generated bundle `dist/plot.umd.js` can then be loaded like so:
 <script src="plot.umd.js"></script>
 ```
 
-Alternatively, you can attach the `dist/plot.umd.js` file to an [Observable notebook](https://observablehq.com) like so:
+Alternatively, you can attach the `dist/plot.umd.js` file to an [Observable notebook](https://observablehq.com), and then load it like so:
 
 ```js
 Plot = require(await FileAttachment("plot.umd.js").url())