docs: Fix spelling (propery -> property)

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

docs/bootindex.txt

@@ -1,4 +1,4 @@
-= 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