YAML CPP: Fix Doxygen warning
Before this change Doxygen would report the following warning:
> found subsection command outside of section context!
.
- infos/metadata =
- infos/description = This storage plugin reads and writes data in the YAML format
+# YAML CPP
+
## Introduction
The YAML CPP plugin reads and writes configuration data via the [yaml-cpp][] library.
