docs: Corrext tx multisign documentation example. (cosmos#10274)

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## Description

Closes: #XXXX

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The docs state:

> Signing with multiple signers is done with the `tx multisign` command.

Yet the examples use an incorrect `multisignsign`:
```shell
simd tx multisignsign ...
```


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Author: paul121

docs/run-node/txs.md

@@ -61,12 +61,12 @@ For example, starting with the `unsigned_tx.json`, and assuming the transaction
 
 ```bash
 # Let signer1 sign the unsigned tx.
-simd tx multisignsign unsigned_tx.json signer_key_1 --chain-id my-test-chain --keyring-backend test > partial_tx_1.json
+simd tx multisign unsigned_tx.json signer_key_1 --chain-id my-test-chain --keyring-backend test > partial_tx_1.json
 # Now signer1 will send the partial_tx_1.json to the signer2.
 # Signer2 appends their signature:
-simd tx multisignsign partial_tx_1.json signer_key_2 --chain-id my-test-chain --keyring-backend test > partial_tx_2.json
+simd tx multisign partial_tx_1.json signer_key_2 --chain-id my-test-chain --keyring-backend test > partial_tx_2.json
 # Signer2 sends the partial_tx_2.json file to signer3, and signer3 can append his signature:
-simd tx multisignsign partial_tx_2.json signer_key_3 --chain-id my-test-chain --keyring-backend test > partial_tx_3.json
+simd tx multisign partial_tx_2.json signer_key_3 --chain-id my-test-chain --keyring-backend test > partial_tx_3.json
 ```
 
 ### Broadcasting a Transaction