pgsql: Fix some grammar in documentation of spgist

Fix some grammar in documentation of spgist

Discussion: /messages/by-id/92961161-9b49-e42f-0a72-d5d47e0ed4de@postgrespro.ru
Author: Liudmila Mantrova
Reviewed-by: Jonathan Katz, Tom Lane, Michael Paquier
Backpatch-through: 9.4

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d93ba74c22c2cdbb978f84d51032486c3e05b5dc

Modified Files
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doc/src/sgml/spgist.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)