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>

docs/qcow2-cache.txt

@@ -15,7 +15,7 @@ not a straightforward operation.
 This document attempts to give an overview of the L2 and refcount
 caches, and how to configure them.
 
-Please refer to the docs/specs/qcow2.txt file for an in-depth
+Please refer to the docs/interop/qcow2.txt file for an in-depth
 technical description of the qcow2 file format.