change gentx cmd amount field to arg from flag (#8183)

* Read gentx amount as argument

* Update gentx command usage

* Update gentx tests

* Update docs of gentx

Co-authored-by: Alessio Treglia <alessio@tendermint.com>

docs/run-node/run-node.md

@@ -53,7 +53,7 @@ Now that your account has some tokens, you need to add a validator to your chain
 
 ```bash
 # Create a gentx.
-simd gentx my_validator --amount 100000stake --chain-id my-test-chain --keyring-backend test
+simd gentx my_validator 100000stake --chain-id my-test-chain --keyring-backend test
 
 # Add the gentx to the genesis file.
 simd collect-gentxs

docs/using-the-sdk/cosmovisor.md

@@ -103,7 +103,7 @@ rm -rf $HOME/.simapp
 ./build/simd keys --keyring-backend=test add validator
 ./build/simd init testing --chain-id test
 ./build/simd add-genesis-account --keyring-backend=test $(./build/simd keys --keyring-backend=test show validator -a) 1000000000stake,1000000000validatortoken
-./build/simd gentx --keyring-backend test --chain-id test validator
+./build/simd gentx --keyring-backend test --chain-id test validator 100000stake
 ./build/simd collect-gentxs
 ```