Fix error in exception message in Markdown Converter [ci skip]

lib/jekyll/converters/markdown.rb

@@ -21,7 +21,7 @@ module Jekyll
             else
               Jekyll.logger.error "Invalid Markdown Processor:", "#{@config['markdown']}"
               Jekyll.logger.error "", "Valid options are [ maruku | rdiscount | kramdown | redcarpet ]"
-              raise FatalException, "Invalid Markdown process: #{@config['markdown']}"
+              raise FatalException, "Invalid Markdown Processor: #{@config['markdown']}"
             end
           end
         @setup = true