docs: s390: Fix wrong label Guest2 instead of Guest3

This fixes:

Documentation/s390/vfio-ap.rst:488: WARNING: duplicate label s390/vfio-ap:guest2, other instance in /home/iha/sdb/opensource/lkmp/linux_doc/Documentation/s390/vfio-ap.rst
Signed-off-by: Vitor Massaru Iha <vitor@massaru.org>
Link: https://lore.kernel.org/r/20200430221238.101838-1-vitor@massaru.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/s390/vfio-ap.rst

@@ -484,7 +484,7 @@ CARD.DOMAIN TYPE  MODE
 05.00ff     CEX5A Accelerator
 =========== ===== ============
 
-Guest2
+Guest3
 ------
 =========== ===== ============
 CARD.DOMAIN TYPE  MODE