Fix `OnCalendar=*-*-* 6,18:00` instead of `OnCalendar=*-*-* 6:00,18:00`

See https://github.com/systemd/systemd/issues/2956 - it appears that
`OnCalendar=*-*-* 6,18:00` and `OnCalendar=*-*-* 6:00,18:00` are
quite different.

Git-Dch: ignore

debian/apt-daily.timer

@@ -2,7 +2,7 @@
 Description=Daily apt activities
 
 [Timer]
-OnCalendar=*-*-* 6:00,18:00
+OnCalendar=*-*-* 6,18:00
 RandomizedDelaySec=12h
 AccuracySec=1h
 Persistent=true