[mlir] Fix/Clarify parts of MLIR toy tutorial chapter 6+7

Summary:
* add missing comma.
* remove "having to register them here" phrasing, since register it
  is what we're doing, which made the comment a bit confusing.
* remove duplicate code.
* clarify link to chapter 3, since "folder" doesn't appear in that
  chapter.

Differential Revision: https://reviews.llvm.org/D75263