Fix invalid JSON in JWT payload

doc/ci/examples/authenticating-with-hashicorp-vault/index.md

@@ -47,7 +47,7 @@ The JWT's payload looks like this:
   "project_id": "22",                            #
   "project_path": "mygroup/myproject",           #
   "user_id": "42",                               # Id of the user executing the job
-  "user_login": "myuser"                         # GitLab @username
+  "user_login": "myuser",                        # GitLab @username
   "user_email": "myuser@example.com",            # Email of the user executing the job
   "pipeline_id": "1212",                         #
   "pipeline_source": "web",                      # Pipeline source, see: https://docs.gitlab.com/ee/ci/yaml/#common-if-clauses-for-rules