doc: application development: fix DT overlay section

Commit f17630ba75
("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>

doc/develop/application/index.rst

@@ -517,7 +517,7 @@ worth going through, especially if you planning to add new configuration
 options.
 
 Experimental features
-*********************
+~~~~~~~~~~~~~~~~~~~~~
 
 Zephyr is a project under constant development and thus there are features that
 are still in early stages of their development cycle. Such features will be