Fix Mermaid drawings not loading on some browsers

As mentioned in https://github.com/knsv/mermaid/issues/291#issuecomment-287673609, the
parameter should be `startOnLoad` instead of `loadOnStart`. In addition, `mermaid.initialize()`
expects to see `startOnLoad` in the `mermaid` hash:

https://github.com/knsv/mermaid/blob/6cd5ffe637d88525b93417bf3954e9c0e5a4deb8/src/mermaid.js#L118

Closes #42052

app/assets/javascripts/render_mermaid.js

@@ -19,7 +19,11 @@ export default function renderMermaid($els) {
 
   import(/* webpackChunkName: 'mermaid' */ 'blackst0ne-mermaid').then((mermaid) => {
     mermaid.initialize({
-      loadOnStart: false,
+      // mermaid core options
+      mermaid: {
+        startOnLoad: false,
+      },
+      // mermaidAPI options
       theme: 'neutral',
     });
 

changelogs/unreleased/sh-fix-mermaid-start-on-load-typo.yml

+---
+title: Fix Mermaid drawings not loading on some browsers
+merge_request:
+author:
+type: fixed