fix traces description

Co-authored-by: Alex Johnson <[email protected]>

Author: archmoj

README.md

@@ -76,7 +76,7 @@ There are two kinds of plotly.js partial bundles:
 1. The official partial bundles that are distributed to `npm` and `CDN` covered by [the dist README](https://github.com/plotly/plotly.js/blob/master/dist/README.md).
 2. The custom ones that could be created by the user, if none of the distributed packages meet your needs.
 
-Use `traces` option to create a custom bundle of desired traces.
+Use the `traces` option to include just the trace types you need.
 ```
 npm run partial-bundle -- --traces scatter,scattergl,scatter3d
 ```