Update model in readme

Author: CyberFlameGO

README.md

@@ -1,6 +1,6 @@
 # GitHub Copilot for Vim and Neovim
 
-GitHub Copilot uses OpenAI Codex to suggest code and entire functions in
+GitHub Copilot uses OpenAI's GPT-4 model to suggest code and entire functions in
 real-time right from your editor.  Trained on billions of lines of public
 code, GitHub Copilot turns natural language prompts including comments and
 method names into coding suggestions across dozens of languages.