Fix the filename exporter written to

fix brave#13255

Auditors: @bsclifton

Test Plan:
1. Set up some bookmarks on a clean profile
2. Menu->Bookmarks->Export bookmarks
3. The default path should be your download path
4. The dialog should have extension option with html
5. Default extension is html
6. Click Save on the dialog.

app/browser/bookmarksExporter.js

@@ -43,7 +43,7 @@ const showDialog = (state) => {
       personal = createBookmarkArray(state)
       other = createBookmarkArray(state, -1, false)
       try {
-        fs.writeFileSync(fileName, createBookmarkHTML(personal, other))
+        fs.writeFileSync(fileNames[0], createBookmarkHTML(personal, other))
       } catch (e) {
         console.log('Error exporting bookmarks: ', e)
       }