Adjust CREATE DOMAIN example for standard_conforming_strings=on.

Noted by Hitoshi Harada.

doc/src/sgml/ref/create_domain.sgml

@@ -189,8 +189,8 @@ CREATE DOMAIN <replaceable class="parameter">name</replaceable> [ AS ] <replacea
 <programlisting>
 CREATE DOMAIN us_postal_code AS TEXT
 CHECK(
-   VALUE ~ '^\\d{5}$'
-OR VALUE ~ '^\\d{5}-\\d{4}$'
+   VALUE ~ '^\d{5}$'
+OR VALUE ~ '^\d{5}-\d{4}$'
 );
 
 CREATE TABLE us_snail_addy (