pgsql: Fix typo in foreign_key.sql

Fix typo in foreign_key.sql

Introduced by eec0040c4bcd.

Author: Chao Li <lic@highgo.com>
Discussion: /messages/by-id/CAEoWx2kKMdtWKQiYNuwG2L41YwHA7G3sUsRfD9esPJwZyX1+Eg@mail.gmail.com
Backpatch-through: 18

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2106fe25a1c025a75effb7fefcbd47c68d4b9914

Modified Files
--------------
src/test/regress/expected/foreign_key.out | 2 +-
src/test/regress/sql/foreign_key.sql | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)