[clang] [analyzer][docs] Fix invalid MyST toctree 'numbered' option after Markdown migration (PR #207217)

[Balázs Benics via cfe-commits]

steakhal wrote:

> I'm completely unfamiliar with this these details of the documentation build process, but I assume that you tested that this change eliminates the warning/error that you observed, and in that case LGTM.

Yes, it works. I had a look at the resulting HTML. And it looked good - I don't think it changed anything but fixed the warning.

https://github.com/llvm/llvm-project/pull/207217