Update the index query on database_lab.md file

What does this MR do and why?

I was working on my onboarding tasks, which included running queries to get familiar with the PostgreSQL console. One of the queries failed exec CREATE INDEX on application_settings USING btree (instance_administration_project_id) because the instance_administration_project_id column doesn't exist. To address this and assist future new starters, I want to update the index query to use an existing column name. This change ensures that the command works correctly and helps new team members complete their onboarding more smoothly.

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