Mention `--baseline-lenient` in the book

The documentation [here](https://bheisler.github.io/criterion.rs/book/user_guide/command_line_options.html) lists these options: - `--save-baseline <name>` will compare against the named baseline, then overwrite it. - `--baseline <name>` will compare against the named baseline without overwriting it. - `--load-baseline <name>` will load the named baseline as the new data set rather than the previous baseline. But there is also `--baseline-lenient` which I found through [this](https://github.com/bheisler/criterion.rs/issues/530) issue, and it's really helpful especially for CI builds!