[DOC] Ignore racc-generated files

Not only `.ry` sources, generated `.rb` files seem duplicate and
unnecessary.

Author: nobu

.rdoc_options

@@ -1,6 +1,6 @@
 exclude:
-  - lib/rdoc/rd/block_parser.ry
-  - lib/rdoc/rd/inline_parser.ry
+  - lib/rdoc/rd/block_parser.r[by]
+  - lib/rdoc/rd/inline_parser.r[by]
   - lib/rdoc/markdown.kpeg
   - lib/rdoc/markdown/literals.kpeg
 op_dir: _site # for GitHub Pages and should match the config of RDoc task in Rakefile