Update xenserver.rst
Update xenserver.rst

Fixed typo.
apache/cloudstack-docs-install,pdion891/cloudstack-docs-install,unodba/cloudstack-docs-install,apache/cloudstack-docs-install,unodba/cloudstack-docs-install,pdion891/cloudstack-docs-install,unodba/cloudstack-docs-install,pdion891/cloudstack-docs-install,apache/cloudstack-docs-install