Update Prysm config.yml in proof-of-stake-devnet.md

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This PR fixes the mismatching Prysm and Geth config. So that the Prysm and Geth will not be out of sync due to different TTD.

The documentation includes an older copy of Prysm config.yml from the repository original post. But genesis.json is provided as a link to the newest version of that repository. Recently, a commit to that repository makes the chain starts from Bellatrix. The commit is not reflected here. This PR aims to avoid this situation from happening.

[nisdas]

@kasey what else needs to be changed in the docs to get this tutorial updated for starting from bellatrix ?

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