docs: fix capitalization and grammar in README

[lxcxjxhx]

Motivation

The README has minor grammar and capitalization issues:

1. Lowercase start: "currently supports:" should start with capital "C"
2. Grammar error: "You can to install" should be "You can install"
3. Outdated model name: Example uses non-existent gpt-5-nano model

Changes

• Capitalized "Currently supports:" in LLM support section
• Fixed grammar: "You can to install" → "You can install"
• Updated example model from gpt-5-nano to gpt-3.5-turbo (real model)

Testing

• Grammar is correct
• Example uses real model name
• Markdown renders correctly

Checklist

• Documentation fix only
• No breaking changes
• Improves clarity and correctness

[lxcxjxhx]

Hi @NVIDIA/garak-maintainers,

This PR fixes capitalization and grammar issues in the README to improve documentation quality.

The DCO check has passed. Could you please review and approve the workflow runs when you have time?

Thank you for your consideration!

[jmartin-tech]

gpt-5-nano is valid and will be active until 2026-12-11 slightly longer than gpt-3.5-turbo which is scheduled for removal 2026-10-23.