doc: fix section heading in device model hld

Fix incorrect section heading level.

Tracked-on: #1668

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

Author: dbkinder

doc/developer-guides/hld/hld-devicemodel.rst

@@ -781,7 +781,7 @@ ACPI Virtualization
 *******************
 
 Introduction
-************
+============
 
 Advanced Configuration and Power Interface (ACPI) provides an open
 standard that operating systems can use to discover and configure