Change 'Average Error Rate' to just 'Error Rate'

It's not really an average with a percentage. 🤔 
[Culprit #1](https://github.com/MC-API/static/blob/master/views/performance.html#L133-L134) and [Culprit #2](https://github.com/MC-API/static/blob/master/views/performance.html#L247-L248)