Fix dbt Connector documentation link in README

[0xroll]

Updated the link to the dbt Connector documentation for accuracy.

pointing to : https://docs.dune.com/api-reference/connectors/dbt/overview#dbt-connector-overview

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Description:

[...]

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quick links for more information:

• README.md
• spellbook docs
• CONTRIBUTING.md

[cursor]

PR Summary

Replaces an outdated docs URL with the correct dbt Connector docs link.

• Updates README.md dbt Connector link to https://docs.dune.com/api-reference/connectors/dbt/overview#dbt-connector-overview

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