Update docs per PR 882

PR 882, [here](https://github.com/nginx/agent/pull/882/files#diff-2c98304c54a9aa70aeeb130d57b954f3186ff3131625b2b698c4c8885fb51311R66), changes config_dirs to `allowed_directories`

The corresponding docs, https://docs.nginx.com/nginx-agent/configuration/configuration-overview/#configure-with-config-files, still refers to `config_dirs`