Keep seeing warnings about missing 'key' prop for DocumentTitle

Not sure if I'm doing something wrong, if this is a problem with this component, or if the issue is in `react-side-effect`, but it's a slightly distracting (if harmless) warning that I can't get rid of:

```
bundle.0ae94dcb667ef563f4ee.js:sourcemap:3890 Warning: Each child in an array or iterator should have a unique "key" prop. See https://fb.me/react-warning-keys for more information.
in div
in DocumentTitle (created by SideEffect(DocumentTitle))
in SideEffect(DocumentTitle)
in NavbarView (created by RouterContext)
in RouterContext (created by Router)
in Router
in Provider
```