Fix an issue with corpus columns in Quickstart

Summary: Fixes T7050. I got the regexp slightly wrong and didn't catch it because it works fine on modern MySQL.

Test Plan: arc unit --everything still passes.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7050

Differential Revision: https://secure.phabricator.com/D11522