Added missing SnippetsFilter in the HTTPRoutes in example

[leonseng]

Proposed changes

HTTPRoutes in example is missing references to SnippetFilters. Updated with references as documented in https://docs.nginx.com/nginx-gateway-fabric/how-to/traffic-management/snippets/

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork