update readme with naming changes

README.md

@@ -27,7 +27,7 @@ local client = xai.create("your-api-key-here")
 Make a chat completion request:
 
 ```luau
-local response = client:responses({
+local response = client:completions({
     model = "grok-4",
     messages = {
         { role = "user", content = "Hello, Grok!" }
@@ -40,7 +40,7 @@ print(response.choices[1].message.content)
 For backward compatibility, you can also use the static method:
 
 ```luau
-local response = xai.responses("your-api-key", request)
+local response = xai.completions("your-api-key", request)
 ```
 
 ## Types