Short document fix

In the CREATE DOMAIN reference page of the current HEAD, it says

---
CREATE DOMAIN us_postal_code AS TEXT
CHECK(  VALUE ~ '^\\d{5}$'
OR VALUE ~ '^\\d{5}-\\d{4}$'
);
---

but I believe it should conform the standard string style now that the
default is standard_conforming_strings = on. I didn't grep if there
other pages like this.

Regards,
-- 
Hitoshi Harada