Change appender-ref to the more updated AppenderRef

[GWphua]

Description

According to the log4j2 javadocs, Appender-ref is an alias for AppenderRef.

I think we should standardize to use AppenderRef in our examples, so as not to confuse the doc readers.

---

Key changed/added classes in this PR

• log4j2.xml configs under examples/

---

This PR has:

• [x] been self-reviewed.