Header notification toggle has an imprecise name

The button to toggle notification display in the header has a label, but this label does not show the expected alternative text.

https://github.com/xwiki/xwiki-platform/blob/da56c0defbd5d810035ce9164dd38ad08984a709/xwiki-platform-core/xwiki-platform-alerts/xwiki-platform-alerts-ui/src/main/resources/XWiki/Alerts/Code/AlertsMenuUIX.xml#L143

We'd expect something along the lines of `Toggle notifications`.