This changes the deprecated :show-nested: instruction to the :nested: one [1].
This also fixes warning about misindented block [2].

[1] https://sphinx-click.readthedocs.io/en/latest/usage/#directive-click

[2]
```
06:22:34  swh fs mount:1:Unexpected indentation.
06:22:34  make: *** [../../swh-docs/Makefile.sphinx:32: sphinx/html] Error 2
```