change this to ctx in v2 signature example

Readme.md

@@ -36,7 +36,7 @@ app.use(async (ctx, next) => {
 })
 
 app.use(async ctx => {
-  const user = await User.getById(this.session.userid) // await instead of yield
+  const user = await User.getById(ctx.session.userid) // await instead of yield
   ctx.body = user // ctx instead of this
 })
 ```