docs: Fix spelling (propery -> property)

Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

docs/bootindex.txt

-= Bootindex propery =
+= Bootindex property =
 
 Block and net devices have bootindex property. This property is used to
 determine the order in which firmware will consider devices for booting