Update sample code (#24)

rmagick/rmagick#1608

Author: Watson1978

demo.rb.html

@@ -28,12 +28,12 @@ <h1>demo.rb</h1>
 begin
   puts 'Read images...'
 
-  model = Magick::ImageList.new('../doc/ex/images/model.miff')
+  model = Magick::ImageList.new('images/model.miff')
   model.border_color = 'black'
   model.background_color = 'black'
   model.cur_image[:Label] = 'RMagick'
 
-  smile = Magick::ImageList.new('../doc/ex/images/smile.miff')
+  smile = Magick::ImageList.new('images/smile.miff')
   smile.border_color = 'black'
   smile.cur_image[:Label] = 'Smile'