proxmox-tape: use correct api call for 'load-media-from-slot'

it's a 'post' api call, not 'put'

reported here:
https://forum.proxmox.com/threads/lto8.109946/
and here:
https://forum.proxmox.com/threads/cant-clear-tape.86454/

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/bin/proxmox-tape.rs

@@ -292,7 +292,7 @@ async fn load_media_from_slot(mut param: Value) -> Result<(), Error> {
     let client = connect_to_localhost()?;
 
     let path = format!("api2/json/tape/drive/{}/load-slot", drive);
-    client.put(&path, Some(param)).await?;
+    client.post(&path, Some(param)).await?;
 
     Ok(())
 }