go.talks/present: put newline after end message

R=r
CC=golang-dev
https://golang.org/cl/6718047
diff --git a/present/static/playground.js b/present/static/playground.js
index ca4d2a7..de3bd95 100644
--- a/present/static/playground.js
+++ b/present/static/playground.js
@@ -32,6 +32,7 @@
       } else {
         s += ".";
       }
+      s += "\n";
       showMessage(o, s, "system");
     }
   }