ox-html.el: Fix overflow css for pre.src HTML tag

* lisp/ox-html.el (org-html-style-default): Fix overflow css
for pre.src HTML tag.

See <https://orgmode.org/list/CAPRwMaYiWVRLB=oTvQv=1JDNdUPxiYMow8MYdqj1sh_udfhUEw@mail.gmail.com>

lisp/ox-html.el

@@ -311,7 +311,7 @@ for the JavaScript code in this tag.
   }
   pre.src {
     position: relative;
-    overflow: visible;
+    overflow: auto;
     padding-top: 1.2em;
   }
   pre.src:before {