doc: Heading for monitor command cpu got lost, restore it

Broken in commit 2313086a.

Signed-off-by: Markus Armbruster <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>
diff --git a/qemu-monitor.hx b/qemu-monitor.hx
index bf47ae0..220ed9c 100644
--- a/qemu-monitor.hx
+++ b/qemu-monitor.hx
@@ -611,6 +611,8 @@
     },
 
 STEXI
+@item cpu @var{index}
+@findex cpu
 Set the default CPU.
 ETEXI