Put prefixPath in config.devServer.devMiddleware.publicPath

After updating from webpack-dev-server v3. -> v.4 the configuration
config.devServer.publicPath is now:

config.devServer.devMiddleware.publicPath

See:

https://github.com/webpack/webpack-dev-server/blob/master/migration-v4.md#-breaking-changes

Avoid error:

ValidationError: Invalid options object. Dev Server has been initialized using an options object that does not match the API schema.

razzle.config.js

@@ -352,7 +352,7 @@ const defaultModify = ({
 
   if (prefixPath) {
     if (target === 'web' && dev) {
-      config.devServer.publicPath = prefixPath;
+      config.devServer.devMiddleware.publicPath = prefixPath;
     }
     const pp = config.output.publicPath;
     config.output.publicPath = `${pp}${prefixPath.slice(1)}/`;