Remove the git option from ruby-prof gem pointing to wycats repo.

According to him, at one point his fork was the one that had the call
stack printer, and it's in the main repo now.

[ci skip]

railties/guides/source/performance_testing.textile

@@ -447,7 +447,7 @@ h4. Using Ruby-Prof on MRI and REE
 Add Ruby-Prof to your applications' Gemfile if you want to benchmark/profile under MRI or REE:
 
 <ruby>
-gem 'ruby-prof', :git => 'git://github.com/wycats/ruby-prof.git'
+gem 'ruby-prof'
 </ruby>
 
 Now run +bundle install+ and you're ready to go.