correct websocket rpc port parameter

[v9n]

There was a typo in ws-port parameter. The document has --rpc-port appear twice. The second time it should be --ws-port.

Originally the doc looks like this

| `--rpc-port <port>` | Specifies the TCP port to use for the HTTP RPC server.
#... other parameter
| `--rpc-port <port>` | Specifies the TCP port to use for the WebSockets RPC server.

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