Update benchmark docs

[giuseongit]

With the 0.28.0 release Time api has changed, Time#now has been deprecated. Use Time#local or Time#utc. Both will work as it is used only to measure a time span.

[pyrsmk]

Furthermore, benchmarks should be relevant. There's no sense about comparing a Ruby gem against a Crystal shard... If you need to benchmark something, just do it with direct HTML parsing competitors.