Fix chokidar on() handler (#3694)

@clarkdo need to merge this asap -- related to https://github.com/nuxt/nuxt.js/pull/3633#discussion_r209436334

lib/builder/builder.js

@@ -644,7 +644,7 @@ export default class Builder {
     )
     this.watchers.restart = chokidar
       .watch(nuxtRestartWatch, options)
-      .on('change', (event, _path) => {
+      .on('change', (_path) => {
         const parsedPath = path.parse(_path)
         restartServer(`${parsedPath.base}${parsedPath.ext}`)
       })