Update index.js.flow (#1)

This was giving syntax error in flow-bin 0.93.0. This declaration fixes the issue

Author: patrikniebur

index.js.flow

@@ -1,3 +1,4 @@
 interface DocumentTitle {}
+declare function useDocumentTitle(title: string): DocumentTitle;
 
-export default function useDocumentTitle(title: string): DocumentTitle;
+export default useDocumentTitle;