desktop: Match the name in the appstream file
With the addition of the appstream file, the app is showing up in Flathub and software apps (like GNOME Software) as "Volume Control", but on the desktop (using the Desktop file), it's showing up differently as "PulseAudio Volume Control", which is confusing.
Like commit be022609 mentions, there are some guidelines for naming apps which prefer the shorter "Volume Control" more than "PulseAudio Volume Control". It also helps for example GNOME users who get an ellipsized name in GNOME Software and the GNOME desktop.
To make sure people still get a hit on "PulseAudio" when searching for the app, let's add that specifically to the tags. That should prevent search from breaking in a lot of cases.