match new location of index.html

[emmyoop]

Description

dbt-core moved the index.html from core/dbt/include to core/dbt/task/docs. Match the new path.

Checklist

• [ ] I have read the contributing guide and understand what's expected of me
• [ ] I have signed the CLA
• [ ] I have run this code in development and it appears to resolve the stated issue
• [ ] I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• [ ] I have run changie new to create a changelog entry