Bump express from 4.16.3 to 4.17.3 in /example

Bumps [express](https://github.com/expressjs/express) from 4.16.3 to 4.17.3.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/master/History.md)
- [Commits](expressjs/express@4.16.3...4.17.3)

---
updated-dependencies:
- dependency-name: express
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

example/package.json

@@ -7,7 +7,7 @@
   "license": "WTFPL",
   "private": false,
   "dependencies": {
-    "express": "^4.16.3",
+    "express": "^4.17.3",
     "gcf-express-app": "^0.0.1"
   }
 }