fix: correct 3 typos in thesis (sill→still, wen→we, a a→a)

[Jah-yee]

Summary

Fix 3 typos reported in issue #4:

1. chapters/rc_future/questions.tex: 'sill' → 'still'

   • Context: "How to tell how far these computer systems are still from genuine human-level reading comprehension?"

2. chapters/openqa/system.tex: 'wen' → 'we'

   • Context: "...it should further improve the system performance if we can collect more training examples."

3. chapters/coqa/dataset.tex: 'a a' → 'a'

   • Context: "...reliable automatic evaluation with a strong human performance."

All three are surgical one-word/phrase fixes exactly as identified in the issue.

PR Checklist

• Changes are minimal and surgical
• GH007 compliant (commit email: 166608075+Jah-yee@users.noreply.github.com)
• No unnecessary changes to other parts of the codebase

[Jah-yee]

Hi! Following up on this PR — it fixes 3 typos reported in issue #4 (sill→still, wen→we, a a→a) in the thesis chapters. The changes are minimal and surgical. Let me know if you'd like any adjustments!