qemu-options: Fix broken sheepdog URL

The sheepdog URL is broken twice: First it uses a duplicated
http:// prefix, second the website seems to have moved to
https://sheepdog.github.io/sheepdog/ instead.
Signed-off-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NJeff Cody <jcody@redhat.com>

qemu-options.hx

@@ -2586,7 +2586,7 @@ Example
 qemu-system-i386 --drive file=sheepdog://192.0.2.1:30000/MyVirtualMachine
 @end example
 
-See also @url{http://http://www.osrg.net/sheepdog/}.
+See also @url{https://sheepdog.github.io/sheepdog/}.
 
 @item GlusterFS
 GlusterFS is a user space distributed file system.