docs: fix incorrect configuration example in README │

Correct the setup configuration example by changing:
- 'api' table to 'connection' table
- 'provider' to 'api_provider'

Author: zzhirong

README.md

@@ -171,8 +171,8 @@ Here is the full list of predefined command actions:
 
 ```lua
 require("codegpt").setup({
-  api = {
-    provider = "openai",  -- or "Ollama", "Azure", etc.
+  connection = {
+    api_provider = "openai",  -- or "Ollama", "Azure", etc.
     openai_api_key = vim.fn.getenv("OPENAI_API_KEY"),
     chat_completions_url = "https://api.openai.com/v1/chat/completions",
   },