Issues in PayloadFactory mediator docs

[GDLMadushanka]

Description

• Payload factory mediator is renamed to "Payload mediator" in VSCode extension. We need the same change in docs.

• In the freemarker tamplate section, we need to rename var to vars

• We have text | XML | JSON support in meditor. definition in doc needs to be changed

<payloadFactory media-type="xml | json" template-type="default | freemarker">
    <format/>
</payloadFactory>

Steps to Reproduce

N/A

Version

4.4.0

Environment Details (with versions)

No response