Update help_bn.md

Author: jimmygchen

book/src/help_bn.md

@@ -32,6 +32,7 @@ FLAGS:
         --disable-deposit-contract-sync        Explicitly disables syncing of deposit logs from the execution node. This
                                                overrides any previous option that depends on it. Useful if you intend to
                                                run a non-validating beacon node.
+        --disable-duplicate-warn-logs          This flag is deprecated and has no effect.
     -x, --disable-enr-auto-update              Discovery automatically updates the nodes local ENR with an external IP
                                                address and port as seen by other peers on the network. This disables
                                                this feature, fixing the ENR's IP/PORT to those specified on boot.