docs: fix broken paths to docs/interop/qcow2.txt

With the move of some docs to docs/interop on d59157ea05,
a reference path was not updated.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
1 file changed