doc-guide: kernel-doc: fix example for nested_foobar struct

There's a missing */ at the end of Kernel docs example.
Even adding it, it will still produce 3 warnings:

	example:33: warning: Function parameter or member 'bar' not described in 'nested_foobar'
	example:33: warning: Function parameter or member 'bar.st1' not described in 'nested_foobar'
	example:33: warning: Function parameter or member 'bar.st2' not described in 'nested_foobar'

So, make the example more complete and add the missing end
of comment there.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>