Bump nodemailer from 6.4.6 to 6.4.16 in /18. Understanding Validation

Bumps [nodemailer](https://github.com/nodemailer/nodemailer) from 6.4.6 to 6.4.16.
- [Release notes](https://github.com/nodemailer/nodemailer/releases)
- [Changelog](https://github.com/nodemailer/nodemailer/blob/master/CHANGELOG.md)
- [Commits](nodemailer/nodemailer@v6.4.6...v6.4.16)

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

Author: dependabot[bot]

18. Understanding Validation/package-lock.json

@@ -34,7 +34,7 @@
     "express-validator": "^6.4.0",
     "mongodb": "^3.5.5",
     "mongoose": "^5.9.6",
-    "nodemailer": "^6.4.6"
+    "nodemailer": "^6.4.16"
   },
   "devDependencies": {
     "nodemon": "^2.0.2"