docs: fix QLoRA paper link

[dajiaohuang]

Summary

Fix the QLoRA paper reference in training/README_en.md so GitHub Markdown renders it as a link. Also restore the space before the following Appendix reference.

Validation

• Static assertion confirms the exact link is rendered with adjacent Markdown delimiters and is followed by a prose space.
• Parsed all 38 tracked Python files and 14 notebooks as source/JSON without executing notebook cells, model code, or training scripts.
• git diff --check is clean.

Scope

Documentation only: one line in the training README. No models, weights, tokenizer, dataset, GPU operation, dependencies, CI, API, or runtime behavior changed.