Documentation: virt: Fix broken reference to virt tree's index

Fix broken reference to virt/index.rst.

Fixes: 2f5947dfcaec ("Documentation: move Documentation/virtual to Documentation/virt")
Signed-off-by: Sheriff Esseson <sheriffesseson@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
1 file changed