Fix the link to known issues

[matkuliak]

Summary of the changes / Why this is an improvement

• It seems that there are currently no issues for JDBC. Does it make sense to remove this mention?

Checklist

• [x] CLA is signed

[mfussenegger]

The label was renamed. The new link would be https://github.com/crate/crate/issues?q=is%3Aopen+is%3Aissue+label%3A%22client%3A+PostgreSQL+JDBC%22+

[matkuliak]

I see. Then consider this a link fix PR. Thank you