pgsql: Specify putting two spaces after sentence-ending period, which is

Log Message:
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Specify putting two spaces after sentence-ending period, which is current
practice also.

Modified Files:
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pgsql/doc/src/sgml:
sources.sgml (r2.28 -> r2.29)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/sources.sgml?r1=2.28&r2=2.29)