Fix publicPath for playground

[epicfaace]

Reasons for making this change

Fixes the publicPath for the playground to "react-jsonschema-form". This way, when the playground is hosted at https://mozilla-services.github.io/react-jsonschema-form, fetching other js files will work. This problem cropped up recently with adding in the Monaco editor, as it generates a few other js worker files.

If this is related to existing tickets, include links to them as well.

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
• I'm adding a new feature
  • I've updated the playground with an example use of the feature