Update git submodules

* Update docs/submodules/doctor from branch 'stable/danube'
  - danube release notes

    Change-Id: Ic2401cc2c3e57324504d2c2d66a69c2e90a4ee91
    Signed-off-by: Ryota MIBU <r-mibu@cq.jp.nec.com>
    (cherry picked from commit 7a14778cf61c8a5000dbfb1600510440cad39e9f)

diff --git a/docs/submodules/doctor b/docs/submodules/doctor
index e827b25..b2b4b7d 160000 (submodule)
--- a/docs/submodules/doctor
+++ b/docs/submodules/doctor
@@ -1 +1 @@
-Subproject commit e827b253fc4d22866775acb3b04a2c20da02019b
+Subproject commit b2b4b7df99c84e07c9e62fd0da286c562ffb7051