Update README.md

[ternera]

I cleaned up some grammatical errors and formatting issues.

Closes # n/a

💸 TL;DR

I found a broken link in the Readme. I changed the security report link to direct to Reddit's Hackerone page for easy access, and I fixed a grammatical error that I noticed. [CI SKIP]

📜 Details

Design Doc

Jira

🧪 Testing Steps / Validation

✅ Checks

• [ ] CI tests (if present) are passing
• [ ] Adheres to code style for repo
• [ ] Contributor License Agreement (CLA) completed if not a Reddit employee