fix: Correct option typo --log-level -> --log_level

[ftnext]

Thank you for organizing the observability documentation.
I found a typo in the logging options,
https://google.github.io/adk-docs/observability/logging/#configuring-logging-with-the-adk-cli
so I'll send a pull request.

% uvx --from google-adk adk web --log-level DEBUG

Error: No such option: --log-level Did you mean --log_level?

$ git grep log-level
docs/observability/logging.md:When running agents using the ADK's built-in web or API servers, you can easily control the log verbosity directly from the command line. The `adk web`, `adk api_server`, and `adk deploy cloud_run` commands all accept a `--log-level` option.
docs/observability/logging.md:adk web --log-level DEBUG path/to/your/agents_dir
docs/observability/logging.md:The available log levels for the `--log-level` option are:

related: #534

[rsamborski]

Thanks. I just stumbled on the same bug ;-)