bittorrentsync: fix storage_path.

If this path is a symlink, btsync won't be able to read it if it's not ending with "/".

As seen in https://github.com/NixOS/nixpkgs/commit/f02d4ec9ed39694e04aa52937a9f73e217aff7f8
Broken in https://github.com/NixOS/nixpkgs/commit/0539ed477151a3adfff5012cabff0527a9a8c0bb