Stop specifying <pre/> margin

Author: leoj3n

prettify.less

@@ -78,11 +78,9 @@ pre, code, pre *, code * {
 }
 
 pre {
-  margin: 0;
   overflow: auto;
   border-radius: 5px;
   padding: 10px 20px;
-  margin-bottom: 20px;
   vertical-align: top;
   border: 1px solid @haze;
   background-color: @clear;