Fix instructions on how to build the doc.

Change-Id: Id9cde46b38a74f0f6ee368fdc43aee8ec6082b5f
diff --git a/docs/howto_build_SDK.txt b/docs/howto_build_SDK.txt
index 4b6507d..abd1ab2 100644
--- a/docs/howto_build_SDK.txt
+++ b/docs/howto_build_SDK.txt
@@ -65,6 +65,7 @@
 
   $ cd ~/my-android-git
   $ . build/envsetup.sh
+  $ lunch sdk-eng
   $ make sdk
 
 This will take a while, maybe between 20 minutes and several hours depending on