docs(readme): correct dev middleware context usage (#1485)

Author: sidakram

README.md

@@ -398,8 +398,8 @@ In order to develop an app using server-side rendering, we need access to the
 [`stats`](https://github.com/webpack/docs/wiki/node.js-api#stats), which is
 generated with each build.
 
-With server-side rendering enabled, `webpack-dev-middleware` sets the `stats` to `res.locals.webpack.devMiddleware.context.stats`
-and the filesystem to `res.locals.webpack.devMiddleware.context.outputFileSystem` before invoking the next middleware,
+With server-side rendering enabled, `webpack-dev-middleware` sets the `stats` to `res.locals.webpack.devMiddleware.stats`
+and the filesystem to `res.locals.webpack.devMiddleware.outputFileSystem` before invoking the next middleware,
 allowing a developer to render the page body and manage the response to clients.
 
 _Note: Requests for bundle files will still be handled by
@@ -433,8 +433,8 @@ app.use(middleware(compiler, { serverSideRender: true }));
 // The following middleware would not be invoked until the latest build is finished.
 app.use((req, res) => {
   const { devMiddleware } = res.locals.webpack;
-  const outputFileSystem = devMiddleware.context.outputFileSystem;
-  const jsonWebpackStats = devMiddleware.context.stats.toJson();
+  const outputFileSystem = devMiddleware.outputFileSystem;
+  const jsonWebpackStats = devMiddleware.stats.toJson();
   const { assetsByChunkName, outputPath } = jsonWebpackStats;
 
   // Then use `assetsByChunkName` for server-side rendering