pgsql: Fix title in reference section

Fix title in reference section

Reported-by: Robert Kahlert
Author: Daniel Gustafsson

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/994a58407c89724917b05a1564dcf1f2f7f3ea9a

Modified Files
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doc/src/sgml/biblio.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)