make `<xxxx>` visiable (#369)

### What problem does this PR solve?


![image](https://github.com/infiniflow/ragflow/assets/106524776/0c526a56-05b1-42f8-8bf5-cb23a97183b8)

make `<xxxx>` visiable
it was misinterpreted as part of the HTML tags

![image](https://github.com/infiniflow/ragflow/assets/106524776/1c42aef0-6989-40c1-b129-47a835b038a7)

Issue link:None

### Type of change

- [ ] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Breaking Change (fix or feature that could cause existing
functionality not to work as expected)
- [x] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Test cases
- [ ] Python SDK impacted, Need to update PyPI
- [ ] Other (please describe):