Fix default headers plugin README (#4414)

plugins/insomnia-plugin-default-headers/README.md

@@ -20,6 +20,7 @@ Headers can be added by setting a `DEFAULT_HEADERS` environment variable.
     "Connection": "close"
   }
 }
+```
 
 Default header can be removed by setting value to null. For example, use folder environment variables to remove authorization header from anonymous calls