Rake update_javascripts

When I do:
rake update_javascripts

I get this:
(in /Users/mischa/webcolon)
rake aborted!
Don’t know how to build task ‘update_javascripts’

I’m planning to use some of the Ajax libraries in the future, so I
thought I’d follow David’s suggestion
(http://weblog.rubyonrails.org/articles/2005/12/13/) and run rake
update_javascripts. Unfortunately it comes up with the above error.
Should this be run in my project’s folder? If no, where should it be
run? If yes, why do I get the error? Does anyone have any ideas?

Thanks in advance.