pgsql: Backport fix for correct quoting in CREATE DOMAIN example.

Log Message:
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Backport fix for correct quoting in CREATE DOMAIN example. Per Robert
Treat.

Tags:
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REL8_0_STABLE

Modified Files:
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pgsql/doc/src/sgml/ref:
create_domain.sgml (r1.20 -> r1.20.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_domain.sgml.diff?r1=1.20&r2=1.20.4.1)