Use <pre> tag for mermaid code blocks (#1450)

Author: deining

layouts/_default/_markup/render-codeblock-mermaid.html

@@ -1,5 +1,5 @@
 {{ .Page.Store.Set "hasmermaid" true -}}
 
-<div class="mermaid">
+<pre class="mermaid">
   {{- .Inner | safeHTML }}
-</div>
+</pre>