Fix task comment to match which file should be changed

[ci skip]

Rakefile

@@ -45,7 +45,7 @@ else
   Rails::API::StableTask.new('rdoc')
 end
 
-desc 'Bump all versions to match version.rb'
+desc 'Bump all versions to match RAILS_VERSION'
 task :update_versions => "all:update_versions"
 
 # We have a webhook configured in GitHub that gets invoked after pushes.