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

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

[2]
```
10:38:38  Warning, treated as error:
10:38:38  /var/lib/jenkins/workspace/DMOD/tests-on-diff/docs/cli.rst:14:Literal block expected; none found.
10:38:38  make: *** [../../swh-docs/Makefile.sphinx:32: sphinx/html] Error 2
10:38:38  make: Leaving directory '/var/lib/jenkins/workspace/DMOD/tests-on-diff/docs'
10:38:38  ERROR: InvocationError for command '/usr/bin/make -I ../.tox/sphinx/src/swh-docs/swh/ -C docs' (exited with code 2)
```