photos-export-dialog: Replace %e with %-d in the default folder name

Piotr Drągrequested to merge
wip/piotrdrag/removespace into master

%e is the day of the month, with a space preceding single digits. %d is two-digit day of the month with leading zeros, but the “-” modifier removes leading zeros.

%-e is functionally identical to %-d, so we can use whichever.

Closes https://gitlab.gnome.org/GNOME/gnome-photos/issues/65

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