mail-server/dovecot.nix: remove trailing slash from dovecot_maildir

[eqyiel]

This is probably harmless, but before this change I was seeing this in my logs (note the //):

Debug: Namespace inbox: type=private, prefix=, sep=/, inbox=yes, hidden=no, list=yes, subscriptions=yes location=maildir:/mnt/home/vmail//ruben/
Debug: maildir++: root=/mnt/home/vmail//ruben, index=, indexpvt=, control=, inbox=/mnt/home/vmail//ruben, alt=

[r-raymond]

Good catch!