Change backintime.desktop in order to use "true" instead of "True"

Discussion (related to the same issue in KTorrent): https://invent.kde.org/network/ktorrent/-/merge_requests/59

common/backintime.desktop

@@ -4,6 +4,6 @@ Name=Backintime Password Cache
 Exec=/bin/sh -c "backintime pw-cache start 2>&1 >/dev/null"
 Comment=Cache passwords for non-interactive Backintime cronjobs
 Icon=gtk-save
-SingleMainWindow=True
+SingleMainWindow=true
 Terminal=false
 Type=Application