[merge demo] Remove layout-breaking css rule

demo/merge.html

@@ -12,7 +12,6 @@
 <script src="../addon/merge/merge.js"></script>
 <style>
     .CodeMirror { line-height: 1.2; }
-    body { max-width: 80em; }
     span.clicky {
       cursor: pointer;
       background: #d70;