ox-html: Add margin to fix overflow visibility problem

* lisp/ox-html.el (org-html-style-default): Add the margin-top
property to pre.src:hover:before so that the programming language is
fully visible.

TINYCHANGE

lisp/ox-html.el

@@ -300,7 +300,7 @@ property on the headline itself.")
     padding: 3px;
     border: 1px solid black;
   }
-  pre.src:hover:before { display: inline;}
+  pre.src:hover:before { display: inline; margin-top: 14px;}
   /* Languages per Org manual */
   pre.src-asymptote:before { content: 'Asymptote'; }
   pre.src-awk:before { content: 'Awk'; }