Fix broken connectors link in README.md

[stan-dot]

Describe the problem you are trying to solve
The link for connectors in the current REAMDE.md is broken. (https://www.tremor.rs/docs/artefacts/connectors)

Describe the solution you'd like
Update the link to the current connectors page.

[devskillz47]

i think documentation might have moved to a separate project (https://www.tremor.rs/) and not sure if this file is still maintained any longer. For this issue, please see https://www.tremor.rs/docs/0.12/reference/connectors.
(on the website, you can also choose different documentation versions depending on what you're interested in)