doc: Fix inconsistent quotes in some jsonb fields

Single quotes are not allowed in json internals, double quotes are.

Reported-by: Eric Mutta
Discussion: https://postgr.es/m/165715362165.665.3875113264927503997@wrigleys.postgresql.org
Backpatch-through: 14

doc/src/sgml/json.sgml

@@ -701,10 +701,10 @@ UPDATE table_name SET jsonb_field[2] = '2';
    assigned value can be placed.
 
 <programlisting>
--- Where jsonb_field was {}, it is now {'a': [{'b': 1}]}
+-- Where jsonb_field was {}, it is now {"a": [{"b": 1}]}
 UPDATE table_name SET jsonb_field['a'][0]['b'] = '1';
 
--- Where jsonb_field was [], it is now [null, {'a': 1}]
+-- Where jsonb_field was [], it is now [null, {"a": 1}]
 UPDATE table_name SET jsonb_field[1]['a'] = '1';
 </programlisting>