chore(deps): bump @koa/router from 15.0.0 to 15.3.1 in /examples/custom-receiver (#2812)

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Author: dependabot[bot]

examples/custom-receiver/package-lock.json

@@ -11,7 +11,7 @@
   },
   "license": "MIT",
   "dependencies": {
-    "@koa/router": "^15.0.0",
+    "@koa/router": "^15.3.1",
     "@slack/bolt": "^4",
     "@slack/logger": "^4.0.0",
     "@slack/oauth": "^3",