docs(readme): remove redundant overview heading

The "Overview" heading was unnecessary since the content below it directly
describes the plugin's features and capabilities. This change improves the
documentation flow by removing redundant hierarchy.

Author: deathbeam

README.md

@@ -16,8 +16,6 @@ https://github.com/user-attachments/assets/8cad5643-63b2-4641-a5c4-68bc313f20e6
 
 </div>
 
-# Overview
-
 CopilotChat.nvim is a Neovim plugin that brings GitHub Copilot Chat capabilities directly into your editor. It provides:
 
 - 🤖 Native GitHub Copilot Chat integration with official model and agent support (GPT-4o, Claude 3.5 Sonnet, Gemini 2.0 Flash, and more)