Update git submodules

* Update docs/submodules/ipv6 from branch 'stable/danube'
  - Fix broken link

    Change-Id: Ia37d1750e4e2e664784e87bce96ca165ad147be7
    Signed-off-by: Bin Hu <bh526r@att.com>
    (cherry picked from commit a3b23660532d314de57a51c98ef30dddf14f5e8f)

diff --git a/docs/submodules/ipv6 b/docs/submodules/ipv6
index a6fe8cb..fe31f8d 160000 (submodule)
--- a/docs/submodules/ipv6
+++ b/docs/submodules/ipv6
@@ -1 +1 @@
-Subproject commit a6fe8cb94cbfa56134baa43d872f15f79bc16e6b
+Subproject commit fe31f8dcc32c2761eda07551fa6bb7f5466ed6ab