[api] Fix filename and update .vercelignore and vercel.json (#4511)

- Fix a filename in the examples API
- Rename .nowignore to .vercelignore
- Rename now.json to vercel.json

.gitignore

@@ -27,3 +27,4 @@ test/lib/deployment/failed-page.txt
 /.env
 /public
 __pycache__
+.vercel

api/examples/list.ts

@@ -9,7 +9,7 @@ export default withApiHandler(async function(
   res: NowResponse
 ) {
   await extract('https://github.com/vercel/vercel/archive/master.zip', '/tmp');
-  const exampleList = summary('/tmp/now-master/examples');
+  const exampleList = summary('/tmp/vercel-master/examples');
 
   const existingExamples = Array.from(exampleList).map(key => ({
     name: key,