Headings out of order in Book activity

Description: When creating a book activity and chapters (without subchapters) the heading hierarchy is as follows

h1 - Course heading

h2 - Activity heading

h3 - Chapter heading

h5 - Table of contents

Unless a subchapter is used within a chapter, there is never an h4 assigned so the hierarchy is missing a heading level.

Replicated in Boost theme.

Steps to replicate:

1. Access a test course and create a book activity with default settings
2. Add a chapter and some content, then save
3. View the chapter and use a browser extension tool like HeadingsMap to check the heading levels.

Observed:

Chapter hierarchy is missing an h4, Table of Contents header skips straight to an h5

Expected:

Chapter hierarchy maintained and Table of Contents is an h4 instead