pgsql: doc: add example of % substitution for connection URIs

doc: add example of % substitution for connection URIs

Reported-by: Zhou Digoal

Discussion: /messages/by-id/20170912133722.25637.91@wrigleys.postgresql.org

Backpatch-through: 10

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d61f5bb7c444255b064a60df782907f7dddad61a

Modified Files
--------------
doc/src/sgml/libpq.sgml | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

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