Update xenserver.rst

Fixed typo.
diff --git a/source/hypervisor/xenserver.rst b/source/hypervisor/xenserver.rst
index a90e3cf..e5b50a2 100644
--- a/source/hypervisor/xenserver.rst
+++ b/source/hypervisor/xenserver.rst
@@ -73,7 +73,7 @@
    already running
 
 .. warning:: 
-   The lack of up-do-date hotfixes can lead to data corruption and lost VMs.
+   The lack of up-to-date hotfixes can lead to data corruption and lost VMs.
 
 
 XenServer Installation Steps