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

Fix broken reference to virt/index.rst.

Fixes: 2f5947df ("Documentation: move Documentation/virtual to Documentation/virt")
Signed-off-by: Sheriff Esseson <sheriffesseson@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/index.rst

@@ -119,7 +119,7 @@ needed).
    spi/index
    w1/index
    watchdog/index
-   virtual/index
+   virt/index
    input/index
    hwmon/index
    gpu/index