xen-blkfront: Apply changed parameter name to the document

Commit 14e710fe ("xen-blkfront: rename indirect descriptor
parameter") changed the name of the module parameter for the maximum
amount of segments in indirect requests but missed updating the
document.  This commit updates the document.
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20200923061841.20531-4-sjpark@amazon.comSigned-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

Documentation/ABI/testing/sysfs-driver-xen-blkfront

-What:           /sys/module/xen_blkfront/parameters/max
+What:           /sys/module/xen_blkfront/parameters/max_indirect_segments
 Date:           June 2013
 KernelVersion:  3.11
 Contact:        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>