fix express.js example (#247)

Should be using `createServerAdapter` adapter to convert to express-compatible handler.
Based on this issue conversation: #246 (comment)

examples/runtimes/express.js

@@ -1,16 +1,19 @@
 import express from 'express'
 import { Router, error, json } from 'itty-router'
 import 'isomorphic-fetch'
+import { createServerAdapter } from '@whatwg-node/server'
 
 const app = express()
 
 const router = Router()
 
-router.get('/', () => 'Success!').all('*', () => error(404))
+router.get('/', () => 'Success!')
+router.all('*', () => error(404))
 
-const handle = (request) => router.handle(request).then(json).catch(error)
+const handle = (request) => router.fetch(request).then(json).catch(error)
 
-app.use(handle)
+const ittyServer = createServerAdapter(handle)
+app.use(ittyServer.handle)
 
 app.listen(3001)
 console.log('listening at https://localhost:3001')