xen/pciback: Fix comment typo

The double `the' is duplicated in the comment, remove one.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220811120918.17961-1-wangborong@cdjrlc.com
Signed-off-by: Juergen Gross <jgross@suse.com>