docs: fix incorrect API call in Nested params README example

[pintaste]

Problem

The "Nested params" section in README.md references a non-existent API method client.chat.responses.create and mixes two incompatible API surfaces:

• client.chat.responses does not exist — Chat only exposes completions (see src/openai/resources/chat/chat.py)
• input=[...] is the Responses API parameter shape
• response_format={"type": "json_object"} is a Chat Completions parameter
• gpt-5.2 is not a real model name

Running the snippet raises AttributeError: 'Chat' object has no attribute 'responses'.

Fix

Switch to client.chat.completions.create with the correct messages= parameter name, a meaningful prompt, and a real model name (gpt-4o).

-response = client.chat.responses.create(
-    input=[
-        {
-            "role": "user",
-            "content": "How much ?",
-        }
-    ],
-    model="gpt-5.2",
-    response_format={"type": "json_object"},
-)
+response = client.chat.completions.create(
+    messages=[
+        {
+            "role": "user",
+            "content": "Can you generate an example json object describing a fruit?",
+        }
+    ],
+    model="gpt-4o",
+    response_format={"type": "json_object"},
+)

Fixes #3264