Update git submodules

* Update docs/submodules/doctor from branch 'stable/danube'
  - [docs] fix link in danube release notes

    Change-Id: Idd02339dd8547359d4e73849b94e071fb6aeaaf1
    Signed-off-by: Ryota MIBU <r-mibu@cq.jp.nec.com>

diff --git a/docs/submodules/doctor b/docs/submodules/doctor
index 5f48662..d4e454f 160000 (submodule)
--- a/docs/submodules/doctor
+++ b/docs/submodules/doctor
@@ -1 +1 @@
-Subproject commit 5f4866298d0ff7c99d954261b06ff65a9782ca2b
+Subproject commit d4e454f33d88fede7a4a0770ad2af9dbd6b606ed