Fix explanation of directory method in the rakefile.doc

The code
```ruby
file "testdata"              do |t| mkdir t.name end
file "testdata/examples"     do |t| mkdir t.name end
file "testdata/examples/doc" do |t| mkdir t.name end
```
is not the same as `directory "testdata/examples/doc"` because the task defined with `file "testdata/examples/doc" do |t| mkdir t.name end` will cause error saying that there is no folder `testdata`. So I think that they are not equal that's why.

They will be also equal with this file task definition: `file "testdata/examples/doc" do |t| mkdir_p t.name end`