Update load-and-performance-testing.md

Performant isn't a word acknowledged by any dictionaries (apart from urban dictionary: "commonly used by Microsofties but not a real word"). Hence, changed to 'performs well'.
Ad hoc shouldn't be hyphenated.
Removed the 2nd insertion of [Distributed Denial of Service attacks](http://en.wikipedia.org/wiki/Denial-of-service_attack)