Fix failing GitHub action

[jayasting98]

This updates the workflow to use the current sample GitHub Actions workflow on the MarkBind user guide.

It was failing due to the nullish coalescing operator used in the following code introduced in a recent MarkBind pull request MarkBind/markbind#2395.

The nullish coalescing operator was introduced in Node.js v14, but the current workflow uses Node.js v10.

[damithc]

Thanks for the fix, @jayasting98