docs(readme): fix Nested params example to call existing chat.completions.create

[kiwigitops]

Fixes #3264.

The "Nested params" example in README.md called client.chat.responses.create(...), which doesn't exist — Chat only exposes completions, so the snippet raised AttributeError: 'Chat' object has no attribute 'responses'. The example also mixed Responses-API parameter shape (input=[...]) with the Chat Completions parameter response_format, so it wouldn't work as a Responses example either.

This change rewrites the snippet as a real Chat Completions call, preserving response_format (which is valid there) so the "nested params are dictionaries, typed using TypedDict" point still lands.

-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-5.2",
+    response_format={"type": "json_object"},
+)