audio: pipewire: update Helvum screenshot

The UI got refactored recently, it looks much nicer. Took the screenshot
from upstream & cropped it to remove some transparent around the
window.

Source:
https://gitlab.freedesktop.org/pipewire/helvum/-/blob/e78d6f5fb4d507155b7b7eaf201fdefdf4f38f92/docs/screenshot.png

Signed-off-by: Théo Lebrun <[email protected]>