doc-guide:kernel-doc.rst: Reference to foobar

In the Function documentation Section of kernel-doc.rst
there is a function_name() function as an example for
how to make a comment about a function.

But at the end of that example there is a reference to foobar
instead of function_name.

I think that should rather be function_name, because that
was the placeholder the whole example was using.

Signed-off-by: Joris Gutjahr <joris.gutjahr@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>