Update runner.js (vuejs-templates#1235)

Add correct file reference

template/test/e2e/runner.js

@@ -20,7 +20,7 @@ devConfigPromise.then(devConfig => {
 .then(() => {
   // 2. run the nightwatch test suite against it
   // to run in additional browsers:
-  //    1. add an entry in test/e2e/nightwatch.conf.json under "test_settings"
+  //    1. add an entry in test/e2e/nightwatch.conf.js under "test_settings"
   //    2. add it to the --env flag below
   // or override the environment flag, for example: `npm run e2e -- --env chrome,firefox`
   // For more information on Nightwatch's config file, see