Fix typo in share/examples/sound/ossinit.h

Event: Advanced UNIX Programming Course (Fall’23) at NTHU.
Pull Request: https://github.com/freebsd/freebsd-src/pull/974

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

sound: Add an example of basic sound application

This is an example demonstrating the usage of the OSS-compatible APIs
provided by the sound(4) subsystem. It reads frames from a dsp node and
writes them to the same dsp node.

MFC after: 2 weeks
Reviewed by: hselasky, bcr
Differential revision: https://reviews.freebsd.org/D30149