Fixing token-economics dead link (#5302)

The page
[2-token-economics](https://research.web3.foundation/en/latest/polkadot/overview/2-token-economics.html)
is a dead link, replacing it with
[token-economics](https://research.web3.foundation/Polkadot/overview/token-economics).

Co-authored-by: Bastian Köcher <git@kchr.de>