Fix overflow pre tag index.md

In the docs, at h2#testing-on-localhost second pre child, the code snippet literally breaks and goes outside pre, div and everything.
The same happens for h2#promise-and-generator-support third pre child, the code snippet breaks and goes outside the pre and everything.

docs/style.css

@@ -73,6 +73,7 @@ pre {
   -webkit-box-shadow: inset 0 0 5px #eee;
   -moz-box-shadow: inset 0 0 5px #eee;
   box-shadow: inset 0 0 5px #eee;
+  overflow: scroll;
 }
 
 code .comment { color: #ddd }