[Bug] Title of TOC hardcoded as "Table of Contents" for PDFs

**Describe the bug**
The text "Table of Contents" is hardcoded in the file "Pdf Builder.cs" so there is no way to translate to another language.

**To Reproduce**
Steps to reproduce the behavior:
1. Build a PDF with TOC
2. Look at the first page of the TOC section.

**Expected behavior**
I was hoping to be able to set all the texts in "templates/mytemplate/token.json".

**Context (please complete the following information):**
- OS: Windows
- Docfx version: [2.77.0.0]