doc: application development: fix DT overlay section

Commit f17630ba
("doc: application: added description of WARN_EXPERIMENTAL setting")
introduced a new section in the Kconfig settings bit about
experimental features.

This broke the structure of the document, however, because it used the
wrong kind of section underline, resulting in a hierarchy that looks
like this:

   Application Configuration
   └── Kconfig Configuration
   Experimental Features
   └── Devicetree Overlays

when it should have looked like this:

   Application Configuration
   ├── Kconfig Configuration
   |   └── Experimental Features
   └── Devicetree Overlays

This falsely make it look like DT overlays are an experimental
feature!

Fix it.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>