Update git submodules

* Update docs/submodules/apex from branch 'master'
  - Merge "Updates preliminary docs for Euphrates"
  - Updates preliminary docs for Euphrates

    Change-Id: I4e3e78e60756824c1294ab067a325311bc2ac8f2
    Signed-off-by: Tim Rozet <trozet@redhat.com>

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 6a4cd2f..15d8c6b 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 6a4cd2fe76fe445394cd92c3f0e9486084b53489
+Subproject commit 15d8c6b0dff0153a5d106d827442e65fe4c9b559