Fix SDC submodule

Looks like the beijing branch was removed from
SDC repo and hence caused all doc verify jobs to
fail. Restoring this submodule to a reachable
commit.

This was the failure reported:
Unable to checkout
'a7c4d61a0f948780cbb7998217474a14c9c2dccd'
in submodule path 'docs/submodules/sdc.git'

Issue-ID: DOC-271
Change-Id: I57e3c06373b369f142aba7096b1455870fd59828
Signed-off-by: Jessica Wagantall <jwagantall@linuxfoundation.org>

diff --git a/docs/submodules/sdc.git b/docs/submodules/sdc.git
index a7c4d61..751ed40 160000 (submodule)
--- a/docs/submodules/sdc.git
+++ b/docs/submodules/sdc.git
@@ -1 +1 @@
-Subproject commit a7c4d61a0f948780cbb7998217474a14c9c2dccd
+Subproject commit 751ed40ff09744eb535e70164787ca826cfb6acf