Changed documentation for switching to the default ruby

Imo the documentation for switching to the default ruby was a little bit misleading.

Always when I want to set a ruby version to be my new default I need to call ´rvm help´ first because I can never remember the correct option. When scrolling up the help list my eyes see this at first: "default :: use rvm set default ruby and system if it hasn't been set.". I always think about this sentence - to understand it right - but I always misunderstand it and want to try something wrong like that ´rvm default ruby-2.0´ ;)

Because of this I changed this sentence now. I hope you agree with me.

Cheers
Robin