Query for Technical Advisory 99561 has a bug

Faizan Qazi (fqazi) commented:

Page: https://cockroachlabs.com/docs/advisories/a99561.html

## What is the reason for your feedback?

[ ] Missing the information I need

[ ] Too complicated

[ ] Out of date

[x] Something is broken

[ ] Other

## Additional details

The query listed on the page has a bug, where it doesn't detect lost data correctly:
` "virtual" != 'true'`
should be
` "virtual" IS NULL'

Jira Issue: DOC-9670