Fix typo in create_index.sql.
Introduced by
578b229718e.
Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/CAEZATCV_CzRSOPMf1gbHQ7xTmyrV6kE7ViCBD6B81WF7GfTAEA@mail.gmail.com
Backpatch-through: 13