Fix in webpack-and-typescript.md code example

Hi, 
This fixes duplicate module rules.
The intention I suppose was to have a rule for `.js` and one for `.ts(x)`
Cheers!

Author: Omar Gaayeb

content/guides/webpack-and-typescript.md

@@ -116,7 +116,7 @@ module.exports = {
      },
      {
        enforce: 'pre',
-       test: /\.js$/,
+       test: /\.tsx?$/,
        use: "source-map-loader"
      }
    ]