FIx doc to reflect optional

This addresses a part of corona-warn-app#701 i.e. corona-warn-app#701 (comment) by implicitly saying that it can also work _with_ health authorities while it can also work _without_ them.
The claim that it never works without them is just no longer true since v2.9.

This is the most simple (and subtle) way to address it. But it fixes the now wrong statement in the doc.
Of course, the main issue to document the whole new feature with more technical details is still open.

event_registration.md

@@ -2,7 +2,7 @@
 
 Presence Tracing - in the Corona-Warn-App (CWA) also referred to as _Event Registration_ - aims at notifying people of a potential SARS-CoV-2 exposure if they have been to the same venue at a similar time as a positively tested individual. It addresses the potential of airborne transmission in spaces with poor ventilation despite maintaining physical distance. As such, it complements BLE-based proximity tracing with the Exposure Notification Framework.
 
-CWA proposes a fully-automated decentral solution for Presence Tracing which works independent of local health authorities and the collaboration of the host of a venue. It integrates into the existing verification processes of CWA to issue warnings. The solution prioritizes the speed of issuing warnings over their accuracy. A higher degree of accuracy would require manual assessment by local health authorities and the respective resources to do so and is currently not in scope. The solution also protects the privacy of both venues and users, as details about a venue such as description or address are not shared with the CWA infrastructure.
+CWA proposes a fully-automated decentral solution for Presence Tracing which can work independent of local health authorities and the collaboration of the host of a venue. It integrates into the existing verification processes of CWA to issue warnings. The solution prioritizes the speed of issuing warnings over their accuracy. A higher degree of accuracy would require manual assessment by local health authorities and the respective resources to do so and is currently not in scope. The solution also protects the privacy of both venues and users, as details about a venue such as description or address are not shared with the CWA infrastructure.
 
 In summary, the proposed solution allows a _host_ to create a venue through CWA. All necessary data about the venue is encoded in a QR code which can be presented on a mobile device or printed out, for example to be posted at the entrance of the venue. An _attendee_ can check in to the venue by scanning the QR code. Check-ins are stored locally on the mobile device and deleted automatically after two weeks.