Fix broken presentation links in README

The Presentations section contains three relative links that do not resolve on the default branch:\n\n- presentrations/Methods%20To%20Bypass%20A%20Web%20Application%20Firewall.pdf\n- presentation/Web%20Application%20Firewall%20Bypassing%20(How%20to%20Defeat%20the%20Blue%20Team).pdf\n- presentation/A%20Forgotten%20HTTP%20Invisibility%20Cloak.pdf\n\nThe corresponding files are present under presentations/. Please update these links so readers can open the referenced documents.