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

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat approved this pull request.

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. 

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