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

[ternera]

Any possibility of getting this merged?

[niedzielski]

thank you!