Update React & Webpack.md

[jucrouzet]

Since Webpack 2.2 (now the last stable version), some breaking changes were introduced in config that affects this example :

• The "empty" extension is now more needed (and forbidden)
• No more "preLoader" options, must use the module rules format

Fixes #495 #462

[RyanCavanaugh]

Thank you for contributing to the TypeScript Handbook!

We are currently working through a large backlog of open PRs. As part of this, we're asking authors of PRs with active merge conflicts to assess if their changes are still needed, and if so, open a new PR.

For transparency, we are also working on a full-scale reimagining of the Handbook (see the TypeScript Roadmap), meaning we are largely closing down work on the current version of the Handbook. Typo fixes are always appreciated, but any new content should likely wait until the new Handbook is available so that it can be more naturally adapted to that format.