Fix error in SolidFill example (#16)

rmagick/rmagick#1469

solidfill.rb.html

@@ -16,7 +16,7 @@ <h1>solidfill.rb</h1>
 # Demonstrate the SolidFill class
 
 fill = Magick::SolidFill.new('orange')
-img = Magick::Image.new(Cols, Rows, fill)
+img = Magick::Image.new(300, 100, fill)
 
 # Annotate the filled image with the code that created the fill.