fix example in README

This commit removes the redundant ellipsis in the example which creates
the customer table. The ellipsis doesn't any any value but rather
detracts the user from trying out the extension for the first time
because they can't just copy and paste the example.

README.md

@@ -42,7 +42,6 @@ create extension pg_jsonschema;
 
 create table customer(
     id serial primary key,
-    ...
     metadata json,
 
     check (