depend on examples in makefile

Makefile

@@ -6,7 +6,7 @@ pkg/matzo_web_bg.wasm: src/lib.rs Cargo.toml Cargo.lock
 dist/main.js: js/*.js package.json pkg/matzo_web_bg.wasm
 	yarn build
 
-out/index.html: index.html build.py
+out/index.html: index.html build.py examples/*.matzo
 	mkdir -p out
 	python3 build.py --only-html