appdata: Fix release 40.0's release date
The transposed month digits make GNOME Software log warnings.
Like these on startup:
Apr 07 19:07:49 gnome-software[2498]: Tried to set invalid release date: 2021-30-20
Apr 07 19:08:06 gnome-software[2498]: Tried to set invalid release date: 2021-30-20
Apr 07 19:08:06 gnome-software[2498]: Tried to set invalid release date: 2021-30-20