docs(guides): update wds options in hot-module-replacement (#1617)

Define the host property of the options object and set it to 'localhost'. The 
webpack-dev-server CLI defaults the host to 'localhost', but `addDevServerEntrypoints` 
from the webpack-dev-server Node.js API does not set a default and leaves 
it undefined, which will eventually throw "Uncaught SyntaxError: The URL 
'http:/sockjs-node' is invalid" in the browser when following the code example.

webpack/webpack-dev-server#1129

Author: caryli

src/content/guides/hot-module-replacement.md

@@ -145,7 +145,8 @@ const webpack = require('webpack');
 const config = require('./webpack.config.js');
 const options = {
   contentBase: './dist',
-  hot: true
+  hot: true,
+  host: 'localhost'
 };
 
 webpackDevServer.addDevServerEntrypoints(config, options);