alsa-gobject: doc: hwdep: fix missing content of introduction element

[GitHub pull_request - opened <github@xxxxxxxxxxxxxxxx>]

alsa-project/alsa-gobject pull request #26 was opened from takaswie:

A commit eb03ec0340b ("hwdep: fulfill documentation for ALSAHwdep") fulfilled documentation for ALSAHwDep but the content of introduction element is missing.

This patch fixes it.

Request URL   : https://github.com/alsa-project/alsa-gobject/pull/26
Patch URL     : https://github.com/alsa-project/alsa-gobject/pull/26.patch
Repository URL: https://github.com/alsa-project/alsa-gobject