fix: examples/express-ex/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-AXIOS-1038255

Author: snyk-bot

examples/express-ex/package.json

@@ -15,7 +15,7 @@
   "license": "Apache-2.0",
   "dependencies": {
     "body-parser": "^1.19.0",
-    "cloudevents": "^3.1.0",
+    "cloudevents": "^4.0.0",
     "express": "^4.17.1"
   }
 }