Update git submodules

* Update docs/submodules/apex from branch 'stable/danube'
  - Updates ODL source for netvirt+vpp

    Change-Id: I7ffb445ba44bfc904240658b0c3a137b44af28e8
    Signed-off-by: Tim Rozet <trozet@redhat.com>
    (cherry picked from commit f88bfbcb62c2a8b3de12a325482146d1118b39f7)

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 37fa680..fc13f72 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 37fa680f248ac6f358b5a03ed431df3ef4aa958a
+Subproject commit fc13f72801b95cf5707554088c8c81c557cda2be