Documentation: soundwire: Ensure that code is inside the code blocks

The way the document is written now, Sphinx renders empty code blocks
followed by the lines that should be inside them.

Unindent the code-block directives to fix this.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>