Fix ssr-caching example (vercel#12180)

cacheable-response.get expects a get(req, res) method signature:
https://www.npmjs.com/package/cacheable-response#get

Before the change:
```
curl -s  -v http://localhost:3000/blog/first 2>&1 | grep HTTP/1.1
> GET /blog/first HTTP/1.1
< HTTP/1.1 404 Not Found
```
After the change:
```
curl -s  -v http://localhost:3000/blog/first 2>&1 | grep HTTP/1.1
> GET /blog/first HTTP/1.1
< HTTP/1.1 200 OK
```

This is a partial fix of vercel#12019 to make the example work. However it doesn't fix the error 
```
(node:62360) UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
``` , which will need a change at server/next-server.ts

See related issue:
vercel#11525 
vercel#11665

examples/ssr-caching/server.js

@@ -10,8 +10,11 @@ const handle = app.getRequestHandler()
 
 const ssrCache = cacheableResponse({
   ttl: 1000 * 60 * 60, // 1hour
-  get: async ({ req, res, pagePath, queryParams }) => {
-    const data = await app.renderToHTML(req, res, pagePath, queryParams)
+  get: async ({ req, res }) => {
+    const data = await app.renderToHTML(req, res, req.path, {
+      ...req.query,
+      ...req.params,
+    })
 
     // Add here custom logic for when you do not want to cache the page, for
     // example when the page returns a 404 status code:
@@ -28,12 +31,10 @@ const ssrCache = cacheableResponse({
 app.prepare().then(() => {
   const server = express()
 
-  server.get('/', (req, res) => ssrCache({ req, res, pagePath: '/' }))
+  server.get('/', (req, res) => ssrCache({ req, res }))
 
   server.get('/blog/:id', (req, res) => {
-    const queryParams = { id: req.params.id }
-    const pagePath = '/blog'
-    return ssrCache({ req, res, pagePath, queryParams })
+    return ssrCache({ req, res })
   })
 
   server.get('*', (req, res) => handle(req, res))