Fix typos

[Dimitrolito]

This PR addresses minor typographical issues and enhances documentation consistency across three ADR files:

1. adr-002-throttle.md:

   • Fixed incorrect article usage: Changed "a edge case scenario" to "an edge case scenario".

2. adr-013-equivocation-slashing.md:

   • Corrected missing hyphenation in compound adjective: Changed "3 year-old" to "3-year-old".

3. adr-016-securityaggregation.md:

   • Fixed incorrect article usage: Changed "an return set" to "a return set".

Changes Summary:

• 3 files updated.
• 3 additions and 3 deletions.

Checklist:

• Documentation changes only, no functional code affected.
• No additional tests required; CI will validate existing tests.
• All changes adhere to repository contribution guidelines.