Simplify code examples for dynamic API routes (#22546)

Deep matching destructuring syntax is hard to understand and read vs. `const { ... } = req.query`.

docs/api-routes/dynamic-api-routes.md

@@ -17,10 +17,7 @@ For example, the API route `pages/api/post/[pid].js` has the following code:
 
 ```js
 export default function handler(req, res) {
-  const {
-    query: { pid },
-  } = req
-
+  const { pid } = req.query
   res.end(`Post: ${pid}`)
 }
 ```
@@ -69,10 +66,7 @@ An API route for `pages/api/post/[...slug].js` could look like this:
 
 ```js
 export default function handler(req, res) {
-  const {
-    query: { slug },
-  } = req
-
+  const { slug } = req.query
   res.end(`Post: ${slug.join(', ')}`)
 }
 ```