minor fixups in "Writing interaction tests"

Author: etpinard

CONTRIBUTING.md

@@ -184,10 +184,11 @@ which shows the baseline image, the generated image, the diff and the json mocks
 To view the results of a run on CircleCI, download the `build/test_images/` and `build/test_images_diff/` artifacts into your local repo and then run `npm run start-image_viewer`.
 
 ### Writing interaction tests
+
 Keep in mind that the interaction coordinates are relative to the top-left corner of the plot, including the margins. To produce a reliable interaction test, 
 it may be necessary to fix the width, height, margins, X axis range and Y axis range of the plot. For example:
 
-```
+```js
 Plotly.newPlot(gd, [{
     x: [1, 1, 1, 2, 2, 2, 3, 3, 3],
     y: [1, 2, 3, 1, 2, 3, 1, 2, 3],