Update CONTRIBUTING.md (#3028)

Have the sample build script exit after the first failed build.
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 5597695..89878a0 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -161,7 +161,7 @@
 flutter/tools/gn --android --runtime-mode=release
 
 cd out
-ls | xargs -n 1 ninja -C
+ls | xargs -n 1 sh -c 'ninja -C $0 || exit 255'
 
 flutter update-packages --upgrade
 ```