README.md: Fix log level example (#370)
README.md: Fix log level example (#370)

Co-authored-by: Carlos Salguero <9bf3df7d86a17fc76bf2f16ed2e27e6c74079a05@percona.com>
Co-authored-by: Denys Kondratenko <faa4497d78e3d5de08f188a10b5855dca5c676e0@users.noreply.github.com>
Percona-Lab/prometheus_mongodb_exporter