docs: fix deprecated function in routing example (#816)

Changed deprecated ```useBody()``` to ```readBody()```.

fixes: #789

docs/content/1.guide/1.introduction/3.routing.md

@@ -39,7 +39,7 @@ export default eventHandler(async event => {
 ```js
 // routes/user.post.ts
 export default eventHandler(async event => {
-  const body = await useBody(event)
+  const body = await readyBody(event)
   // TODO: Handle body and update user
   return `User updated!`
 })