[MUSIC][AUDIOBOOKS] Fix last chapter duration

[the-black-eagle]

Description

Fixes an issue with the last chapter of m4b audiobooks where the duration is either missing or wrong.

Motivation and context

Forgot originally to convert seconds to milliseconds for the duration of the last chapter.

How has this been tested?

Scanned some m4b audiobooks and observed that the last chapter in all cases has the correct duration now.

What is the effect on users?

m4b books display the correct duration for the last chapter.

Screenshots (if appropriate):

Before :

After :

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• Student submission (PR was done for educational purposes and will be treated as such)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[neo1973]

I don't have audio books to test but it looks like a proper fix 👍