No main class found in mdoc 2.3.5

Hi,

mdoc CLI stopped working since the last release:

```bash
➜  ~ coursier launch org.scalameta:mdoc_2.12:2.3.4 -- --site.VERSION 1.0.0
info: Compiling 1 file to /Users/mbovel/out
info: Compiled in 0.16s (0 errors)
➜  ~ coursier launch org.scalameta:mdoc_2.12:2.3.5 -- --site.VERSION 1.0.0
Cannot find default main class. Specify one with -M or --main-class.
```