Add lettre NVD reference

Author: cookesan

advisories/github-reviewed/2021/07/GHSA-qc36-q22q-cjw3/GHSA-qc36-q22q-cjw3.json

@@ -40,6 +40,10 @@
       "type": "WEB",
       "url": "https://github.com/lettre/lettre/security/advisories/GHSA-qc36-q22q-cjw3"
     },
+    {
+      "type": "ADVISORY",
+      "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-38189"
+    },
     {
       "type": "WEB",
       "url": "https://github.com/lettre/lettre/pull/627/commits/93458d01fed0ec81c0e7b4e98e6f35961356fae2"
@@ -66,4 +70,4 @@
     "github_reviewed_at": "2021-06-01T20:41:39Z",
     "nvd_published_at": "2021-08-08T06:15:00Z"
   }
-}
+}