cloudformation: fix missing comma in 'DATABASE' output

Happened to notice this while I was in here, mostly a cosmetic fix.

cloudformation_templates/edx-reference-architecture.json

@@ -3135,7 +3135,7 @@
                 "Endpoint.Address"
               ]
             },
-            "'\n",
+            "',\n",
             "        'PORT': '",
             {
               "Fn::GetAtt":[