autonumber is broken for sequenceDiagram

[Artur Glier]

Hi! :) First of all - many thanks for adding support for mermaid.js to mdBook! :)

I've been using mdbook to build a documentation for my project and I noticed that autonumber is broken for sequenceDiagram in version 8.8.4 of provided mermaid.min.js (not a js-ninja, might be wrong).

Example:

```mermaid
sequenceDiagram
    autonumber
    participant x1 as Foo
    participant x2 as Bar
    x1 --> x2: FooBar
```

I've replaced the mermaid.min.js file with https://cdn.jsdelivr.net/npm/mermaid/dist/mermaid.min.js (which I think is 8.9.0) and now it works.

Hope this helps someone ;)