server pull: fix comment w.r.t. initial downloaded chunk capacity

server pull: fix comment w.r.t. initial downloaded chunk capacity

> The hash set will be able to hold at least capacity elements
> without reallocating. If capacity is 0, the hash set will not
> allocate.
-- rustdoc, HashSet::with_capacity

So, the number we pass is the amount of chunk "IDs" we safe, which is
then 64Ki, not 16Ki and thus the size we can reference too is also
256 GiB, not 64 GiB.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>