Fix home.sky's #! to modern style

TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/844443004

engine/src/flutter/examples/home.sky

@@ -1,4 +1,4 @@
-#!mojo mojo:sky
+#!mojo:sky_viewer
 <div><!-- remove this element once Document can have multiple element children -->
 <style>
  .h1 { font-size: 2em; margin: 1em; }