Fix terminology: Change "first-order functions" to "first-class functions"

[yyh-gl]

Hi, maintainers.
Thank you for your daily maintenances.

Summary

This PR corrects the terminology from "first-order functions" to "first-class functions".

Changes

• Fix terminology
  • “first-corder function” → “first-class function”

Motivation

I think “first-class” is correct in this context, not “first-order”.

“first-class” is used elsewhere in the specification and other document.