markdown: panic via doc line starting with `}`

[kavoc-brown]

wit-bindgen-markdown panics when a doc-comment line begins with } and is printed inside an indented doc block (e.g. a record field doc).

poc.wit

package a:b;

world w {
  export x: interface {
    record r {
      /// }
      f: u32,
    }
  }
}

Crash:

thread 'main' panicked at crates/core/src/source.rs:102:9:
attempt to subtract with overflow

Other backends do not crash on this WIT.

wit-bindgen version: 0.50.0

OS: Ubuntu 22.04.5 LTS (x86_64)