docs: Fix incorrect Devcon year Update index.md

[0xminds]

Description

I noticed a small typo in the 2024 section under the Execution upgrade naming table.
For the Cancun upgrade, the Devcon year was listed as 2018, but it should be 2019.

Corrected.

P.S. Thanks for Ethereum!

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[corwintines]

Thanks for bringing this up, but I think most of these days are actually wrong based on the archive https://devcon.org/en/past-events/

I can update these dates.

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