fixed namespace in README

[krtschmr]

 rails db:parallel:prepare RAILS_ENV=test
rails aborted!
Don't know how to build task 'db:parallel:prepare' (See the list of available tasks with `rails --tasks`)
Did you mean?  parallel:prepare

[willbryant]

I can't reproduce that behavior, what versions of this gem & rails do you have?

 $ be rake db:parallel:prepare
 $ be rails db:parallel:prepare
 $ be rake parallel:prepare
rake aborted!
Don't know how to build task 'parallel:prepare' (See the list of available tasks with `rake --tasks`)
Did you mean?  db:parallel:prepare
/Users/will/.asdf/installs/ruby/3.2.2/bin/bundle:25:in `load'
/Users/will/.asdf/installs/ruby/3.2.2/bin/bundle:25:in `<main>'
(See full trace by running task with --trace)
 $ be rails parallel:prepare
rails aborted!
Don't know how to build task 'parallel:prepare' (See the list of available tasks with `rails --tasks`)
Did you mean?  db:parallel:prepare

(See full trace by running task with --trace)