Documentation: KUnit: Fix usage bug

Fix a bug of kunit documentation.
Link: https://bugzilla.kernel.org/show_bug.cgi?id=205773

: Quoting Steve Pfetsch:
:
: kunit documentation is incorrect:
: https://kunit.dev/third_party/stable_kernel/docs/usage.html


: struct rectangle *self = container_of(this, struct shape, parent);
:
:
: Shouldn't it be:
: struct rectangle *self = container_of(this, struct rectangle, parent);
: ?

Signed-off-by: Akira Kawata <akirakawata1@gmail.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>