Docs: Add missing 'w' (week) unit to SAMPLE BY reference (Hacktoberfest)

[Hussain-Tinwala]

Fixes #57
What changed: I located the SQL reference documentation for SAMPLE BY and added the missing time unit 'w' (week) to the units table in sample-by.md.
Value: This corrects a factual inaccuracy and provides immediate value to users needing weekly aggregation.
Hacktoberfest: This is my contribution for Hacktoberfest 2025.

[CLAassistant]

All committers have signed the CLA.

[javier]

Thanks for the contribution. As explained at the Haktoberfest blog post, we are not looking for typos or small doc changes for Haktoberfest.

We value quality over quantity. Small typo fixes are always appreciated, but for Hacktoberfest we encourage contributors to look for issues that have a real impact. You will find open issues with the hacktoberfest label in our GitHub repos.