Fix build command for demo app

Test: N/A
Change-Id: I2f46a7ea1aa91a1c90de8e82518cbcc5e797751a
diff --git a/demo/README.md b/demo/README.md
index 43d2ebc..c5c87d8 100644
--- a/demo/README.md
+++ b/demo/README.md
@@ -3,7 +3,7 @@
 ## Building
 
 ```
-TARGET_BUILD_APPS=MicrodroidDemoApp m apps_only dist
+UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true TARGET_BUILD_APPS=MicrodroidDemoApp m apps_only dist
 ```
 
 ## Installing