Only use named imports of react-helmet (#458)

The default export will be removed in version 6.0.0
and is currently missing in @types/react-helmet@5.0.11.

src/components/Pages/Structure.tsx

@@ -1,5 +1,5 @@
 import React from 'react';
-import Helmet from 'react-helmet';
+import {Helmet} from 'react-helmet';
 import Favicon from 'react-favicon';
 
 import styles from './Structure.scss';