Fix: Correct Usage of CleanWebpackPlugin (styleguidist#1565)

I was still using an option that no longer exists. Fixed to mimic the original behavior. The option `root` was removed in v2, but wasn't changed during the upgrade in styleguidist@9804259

It's deleting everything in the styleguidistDir folder which isn't the way it was before.
After this change it only deletes files in build/.

Author: Lazyuki

src/scripts/make-webpack-config.ts

@@ -98,7 +98,7 @@ export default function(
 			},
 			plugins: [
 				new CleanWebpackPlugin({
-					root: config.styleguideDir,
+					cleanOnceBeforeBuildPatterns: [`${config.styleguideDir}/build/**/*`],
 					verbose: config.verbose === true,
 				} as any),
 			],