readme: Clarify secret service D-Bus access command

Clarify the difference for the command between the beta and the nightly
versions.

README.md

@@ -136,9 +136,11 @@ backend interface is running, and you need to allow Fractal to access the D-Bus
 command:
 
 ```sh
-flatpak override --user --talk-name=org.freedesktop.secrets org.gnome.Fractal.Devel
+flatpak override --user --talk-name=org.freedesktop.secrets org.gnome.Fractal
 ```
 
+_For the nightly version, change the application name to `org.gnome.Fractal.Devel`._
+
 Or with [Flatseal](https://flathub.org/apps/details/com.github.tchx84.Flatseal), by adding
 `org.freedesktop.secrets` in the **Session Bus** > **Talk** list of Fractal.