Update git submodules

* Update docs/submodules/vswitchperf from branch 'stable/danube'
  - Fixed indexes

    Change-Id: I5e10d9b2469ed7ec7fbced12b041e659bd568c2d
    Signed-off-by: Trevor Cooper <trevor.cooper@intel.com>

diff --git a/docs/submodules/vswitchperf b/docs/submodules/vswitchperf
index 8a75da0..f2c4cd6 160000 (submodule)
--- a/docs/submodules/vswitchperf
+++ b/docs/submodules/vswitchperf
@@ -1 +1 @@
-Subproject commit 8a75da0f109a6a0ee5b6b347ceab367727f9640c
+Subproject commit f2c4cd6b9512ff77a763404a52188da16277f2a4