[Bug] PDF Header/Footer template path parsing

When specifiying a relative path to an HTML file containing the footer/header path, it instead puts the path string itself as the header/footer in the PDF.

My file structure:
```
/Docfx
├── docfx.json
├── pdf_templates/
│ └── header.html
└── _site/
```

Image of the bug:

**To Reproduce**
Steps to reproduce the behavior:
1. Create an HTML template
2. Link it in the docfx.json ("pdfHeaderTemplate": "pdf_templates/header.html")
3. Open the PDF and see

**Expected behavior**
Expected to see the provided Footer/Header template

**Context**
- OS: Windows
- Docfx version: Nugget 2.77.0