fix(docs): Use nbd-http-server not nbd-server

Thoses options are not present in nbd-server.
I noticed the mistake, while testing nbdkit.

Relate-to: XCPNG-3564
Signed-off-by: Philippe Coval <philippe.coval@vates.tech>

Author: rzr

README.md

@@ -30,7 +30,7 @@ Execute on the first host:
 And on the secondary:
 ```
 # Start a NBD server
-> nbd-server --socket-path ./socket --nbd-name heartbeat --urls "http://totor-a:8000,http://totor-b:8000"
+> nbd-http-server --socket-path ./socket --nbd-name heartbeat --urls "http://totor-a:8000,http://totor-b:8000"
 nbdkit: debug: TLS disabled: could not load TLS certificates
 nbdkit: debug: registering plugin /root/http-nbd-transfer/multi-http-plugin.so
 nbdkit: debug: registered plugin /root/http-nbd-transfer/multi-http-plugin.so (name multihttp)