xen: hypercall.h: fix duplicated word

Change the repeated word "as" to "as a".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: xen-devel@lists.xenproject.org
Link: https://lore.kernel.org/r/20200726001731.19540-1-rdunlap@infradead.org


Signed-off-by: Juergen Gross <jgross@suse.com>