replace element->part

To uniform word like other plugins of `tag_parts` placeholder.

* https://github.com/tagomoris/fluent-plugin-forest
* https://github.com/sonots/fluent-plugin-record-reformer
* https://github.com/fluent/fluent-plugin-rewrite-tag-filter
Merge pull request #58 from y-ken/change-word-elements-to-parts

Replace element->part at the page of Match Pattern