Update example blueprint

[rkoron007]

I was just running through the docs and found a few small errors that prevented me from deploying this, so fixing them! The things were:

1. Using up-to-date plan names (e.g., standard -> free)
2. Updating the env key to runtime
3. Nailing down Render's node version to ensure it worked with the React app.
4. Updating the previewPlan attribute, because it doesn't accept free.

I was able to deploy this blueprint with these updates!

[rkoron007]

It wouldn't let me use the free plan for previews, so I made it basic-256mb.

This was the error:

databases[0]
free plan is not valid as a previewPlan
services[0].previews.plan
free plan is not valid as a previewPlan