Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Bugfix: fix ubuntu image name typo

    Change-Id: I8722f2ddd096bc784f45d4bcf58ea6a84d319fb2
    Signed-off-by: JingLu5 <lvjing5@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index b2fb23c..571ee6a 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit b2fb23ca8ccc1acedaa9156552af4ba347f24103
+Subproject commit 571ee6afca03120c393def421eb35a466f9a940b