.set no longer exists, updated to the existing append (#1682)

Author: jonmchan

docs/testing.md

@@ -34,7 +34,7 @@ It is beneficial to use the same webpack configuration file (generated by webpac
 // config/webpack/test.js
 const environment = require('./environment')
 environment.plugins.get('Manifest').opts.writeToFileEmit = process.env.NODE_ENV !== 'test'
-environment.loaders.set('istanbul-instrumenter', {
+environment.loaders.append('istanbul-instrumenter', {
   test: /\.ts$/,
   enforce: "post",
   loader: "istanbul-instrumenter-loader",