Update getting-started.md

[Olshansk]

Update submodules to make solidity contracts are downloaded

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🔨 Latest commit	9d27c81a796efc419500a917f0f3682e59614c1d

[sbvegan]

Thank you for your contribution, I'm sorry I didn't see this early. This is fixed in https://github.com/ethereum-optimism/stack-docs/pull/28