Fix outdated CLI syntax in `using_the_client.md`

Problem:
Documentation shows outdated CLI syntax that fails in Rucio 38.5.1:

rucio -a=root -S=OIDC -v whoami  # Error: No such option: -a

https://github.com/rucio/documentation/blob/4d8b8a1da3240e16a895ba7b0f0323abc159906e/docs/user/using_the_client.md?plain=1#L122-L124
ff.

Logs:

[root@rucio-server-8658d7545c-drzb5 tmp]# rucio --version
Unable to load schema module rucio_it_tools.rucio_it_sme_policy.schema from policy package, falling back to generic
rucio 38.5.1
[root@rucio-server-8658d7545c-drzb5 tmp]# rucio -a=root -S=OIDC -v whoami
Unable to load schema module rucio_it_tools.rucio_it_sme_policy.schema from policy package, falling back to generic
Usage: rucio [OPTIONS] COMMAND [ARGS]...
Try 'rucio -h' for help.

Error: No such option: -a

Question: How do we prevent docs from becoming outdated?

Potential Solutions:

1. Automated CLI testing in CI - Run documented commands as part of CI
2. Version-specific docs - Separate doc branches per major release
3. Breaking change checklist - Require doc updates in PRs that change interfaces
4. Executable examples - Use integration tests as living documentation

Immediate Action Needed:

• Fix current CLI examples to match v38.5.1 syntax
• Decide on long-term documentation maintenance strategy

Should we create issues for:

1. Fixing the outdated CLI examples?
2. Implementing automated doc validation in CI?