pgsql: Doc: fix meaning of acronym "btree".

Doc: fix meaning of acronym "btree".

Acronym "btree" better means "multi-way balanced tree" rather than
"multi-way binary tree".

Discussion: /messages/by-id/20190105.183532.1686260542006440682.t-ishii@sraoss.co.jp

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6ce6d983224a8cf00ff34bb3dd6c9d734d9bae86

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