Update git submodules

* Update docs/submodules/ipv6 from branch 'stable/euphrates'
  - Release-Ready Documentation

    Change-Id: I2b2b2363c9625f6073d241f1c32ae84c9c31a466
    Signed-off-by: Bin Hu <bh526r@att.com>
    (cherry picked from commit 7a25e49c97b6a05d0a16eee4991723d80d5a69e7)

diff --git a/docs/submodules/ipv6 b/docs/submodules/ipv6
index e8fa336..957d5f2 160000 (submodule)
--- a/docs/submodules/ipv6
+++ b/docs/submodules/ipv6
@@ -1 +1 @@
-Subproject commit e8fa336e23463609c03c17ad0f50cbf09c1ae4bd
+Subproject commit 957d5f277797d6bc078ea19f865bcf718c96985b