findSelectedPageNextPrev: remove dir requirement

[chr0n1x]

this was written before the distinction between Curriculum vs Course was made dirname no longer exists in the context where a course is an entire project

Motivation and Context

Should fix this issue:

How Has This Been Tested?

Locally

Screenshots (if appropriate):

See above.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.