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>

src/server/pull.rs

@@ -586,7 +586,7 @@ pub async fn pull_group(
 
     let mut remote_snapshots = std::collections::HashSet::new();
 
-    // start with 16384 chunks (up to 65GB)
+    // start with 65536 chunks (up to 256 GiB)
     let downloaded_chunks = Arc::new(Mutex::new(HashSet::with_capacity(1024 * 64)));
 
     progress.group_snapshots = list.len() as u64;