Add missing i18n string (#1161)

src/locales/en/messages.json

@@ -1775,6 +1775,9 @@
   "leaveOrganizationConfirmation": {
     "message": "Are you sure you want to leave this organization?"
   },
+  "leftOrganization": {
+    "message": "You have left the organization."
+  },
   "ssoKeyConnectorUnavailable": {
     "message": "Unable to reach the key connector, try again later."
   }