Update Rust Client link to point to the official Lumina repo

[cmwaters]

The Rust Client (Lumina) is now an official client and the documentation should reflect this.

Please update the link for the Rust Client to point to the official repository:
https://github.com/celestiaorg/lumina

This ensures users are directed to the correct and maintained repo.

[cmwaters]

Ref here: https://docs.celestia.org/build/rpc/clients/