Update Custom attachment callback example

The "Custom attachment callback" field in the "Send a custom email" built-in script, is showing examples with wrong "MailAttachment" import class.

 This is the wrong import class.

import com.onresolve.scriptrunner.canned.jira.workflow.postfunctions.MailAttachment

 And this is the right import class.

import com.onresolve.scriptrunner.canned.jira.workflow.postfunctions.mail.MailAttachment