doc: update title in L2 switching user guide

Other use cases use "Ethernet Connection" as the section tite, so update
L2 switching to be consistent.

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Change-Id: Icdd270e994194da998518fe48832c413dfd7d4f4