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

arch/x86/include/asm/xen/hypercall.h

@@ -82,7 +82,7 @@ struct xen_dm_op_buf;
  *     - clobber the rest
  *
  * The result certainly isn't pretty, and it really shows up cpp's
- * weakness as as macro language.  Sorry.  (But let's just give thanks
+ * weakness as a macro language.  Sorry.  (But let's just give thanks
  * there aren't more than 5 arguments...)
  */