adding full class name to mkdir_p method

Change made to fix a NoMethodError: undefined method `mkdir_p`.

NoMethodError: undefined method `mkdir_p' for ModalDiagrams:Module
.../vendor/gems/modaldiagrams-1.1.5/lib/modaldiagrams/modaldiagrams.rb:170:in `generate'
.../vendor/gems/modaldiagrams-1.1.5/lib/tasks/diagrams.rake:11:in `block (2 levels) in <top (required)>'
Tasks: TOP => db:diagrams:pdf => db:diagrams:ps => db:diagrams
(See full trace by running task with --trace)