[sr-dev] [kamailio/kamailio] mod kazoo: wrong kazoo_publish example (#2627)

### Description

Example 1.21. kazoo_publish usage contains string ``` $var(amqp_payload_request) = "{'Event-Category' : 'directory', 'Event-Name' : 'reg_success', 'Contact' : '" + $var(fs_contact) + "', 'Call-ID' : '" + $ci + "', 'Realm' : '" + $fd +"', 'Username' : '" + $fU + "', 'From-User' : '" + $fU + "', 'From-Host' : '" + $fd + "', 'To-User' : '" + $tU +"', 'To-Host' : '" + $td + "', 'User-Agent' : '" + $ua +"' ," + $var(register_contants)+ " }"; ```

This example did not work because the string generates broken JSON: 1. here present value ` $var(register_contants)` but key is not present 2. JSON key and value must be started and terminated by `"` symbol (not by `'`)