Fix typo in SP-GiST error message
Error message didn't match the actual check. Fix that. Compression of leaf
SP-GiST values was introduced in 11. So, backpatch.
Discussion: https://postgr.es/m/
20180810.100742.
15469435.horiguchi.kyotaro%40lab.ntt.co.jp
Author: Kyotaro Horiguchi
Backpatch-through: 11