change fail2ban_loglevel description in README

[jdlourenco]

fail2ban loglevel configuration parameter format has been changing in recent versions.

This pull request aims at using the most common format since version 0.9.0, which is string.

< 0.8.x : set log level using numeric only [ 0...3]
0.9.x : set log level using string only
> 0.10 : Set log level with both numeric and string (python log level)

fail2ban/fail2ban#2008 (comment)

Current version 0.10.0 supports both string and numeric, but the default version on some systems (e.g. ubuntu 16..04) e still 0.9.x therefore using the string format is more compatible.

Best regards,
José Lourenço