fix lodestar's data dir flag name

[Inspector-Butters]

the "Spin up your own Ethereum node" page's instruction for running a lodestar client has a wrong flag for dataDir (it says rootDir).

the correct flag is dataDir : https://chainsafe.github.io/lodestar/run/beacon-management/beacon-cli#--datadir

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[wackerow]

❤️ Great, thanks for the patch @Inspector-Butters! Appreciate the documentation to confirm

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[wackerow]

@all-contributors please add @Inspector-Butters for maintenance

[allcontributors]

@wackerow

I've put up a pull request to add @Inspector-Butters! 🎉