pgsql: Fix typo in sslmode documentation

Fix typo in sslmode documentation

Per bug #6089, noted by Sidney Cadot

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/ed70eaca971c5e30b99ab1e596bce7db3c3fc44f

Modified Files
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doc/src/sgml/libpq.sgml | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)