security-hardening-with-openid-connect: Add missing comma

The JSON snippet was missing a comma after the `runner_environment` value, making the snippet invalid JSON.

Author: Turbo87

content/actions/security-for-github-actions/security-hardening-your-deployments/about-security-hardening-with-openid-connect.md

@@ -78,7 +78,7 @@ The following example OIDC token uses a subject (`sub`) that references a job en
   "run_id": "example-run-id",
   "run_number": "10",
   "run_attempt": "2",
-  "runner_environment": "github-hosted"
+  "runner_environment": "github-hosted",
   "actor": "octocat",
   "workflow": "example-workflow",
   "head_ref": "",