fix(docs/pdk): Rename plain to plaintext for syntax highlighting

[lena-larionova]

Summary

Replacing plain with plaintext, as the docs code syntax highlighter doesn't recognize plain and causes the generated codeblocks on the docs site to break on scroll.

(source: https://docs.konghq.com/gateway/latest/plugin-development/pdk/kong.log/)

Issue reported in docs slack channel.

Checklist

• [ N/A ] The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - Fix: Replace plain with plaintext for syntax highlighting docs.konghq.com#7196

Issue reference

Fix #[issue number]