Update good-practices.md (wrong path)

[nseguias]

Fix error in the path for messages in Good Practices

currently:

use contract::contract::{ContractExecMsg, ContractQueryMsg, InstantiateMsg};

should be:

use contract::contract::sv::{ContractExecMsg, ContractQueryMsg, InstantiateMsg};

[jawoznia]

Thanks for pointing this out.

Also consider checking the https://docs.cosmwasm.com/sylvia as sylvia-book will be deprecated.