Fix default values of buffer plugins

Please check my changes.
Thank you.

plugin     | language | buffer_chunk_limit | buffer_queue_limit | URL
---------- | -------- | ------------------ | ------------------ | ----------------------------------------------
buf_memory | en       | 8m                 | 64                 | http://docs.fluentd.org/articles/buf_memory
buf_memory | ja       | 8m                 | 256                | http://docs.fluentd.org/ja/articles/buf_memory
buf_file   | en       | 256m               | 256                | http://docs.fluentd.org/articles/buf_file
buf_file   | ja       | 8m                 | 256                | http://docs.fluentd.org/ja/articles/buf_file

class                | buffer_chunk_limit | buffer_queue_limit | URL
-------------------- | ------------------ | ------------------ | -----------------------------------------------------------------------------
Fluent::BasicBuffer  | 8m                 | 256                | https://github.com/fluent/fluentd/blob/master/lib/fluent/buffer.rb
Fluent::MemoryBuffer | -                  | 64                 | https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/buf_memory.rb
Fluent::FileBuffer   | -                  | -                  | https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/buf_file.rb