docs: replace deprecated llama 3 model with sonar in ChatPerplexity example (#31716)

**Description:** Updates ChatPerplexity documentation to replace
deprecated llama 3 model reference with the current sonar model in the
API key example code block.

**Issue:** N/A (maintenance update for deprecated model)

**Dependencies:** No new dependencies required

---------

Co-authored-by: Mason Daugherty <[email protected]>

Author: w3bdesign

docs/docs/integrations/chat/perplexity.ipynb

@@ -106,9 +106,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "chat = ChatPerplexity(\n",
-    "    temperature=0, pplx_api_key=\"YOUR_API_KEY\", model=\"llama-3-sonar-small-32k-online\"\n",
-    ")"
+    "chat = ChatPerplexity(temperature=0, pplx_api_key=\"YOUR_API_KEY\", model=\"sonar\")"
    ]
   },
   {
@@ -132,7 +130,7 @@
    },
    "outputs": [],
    "source": [
-    "chat = ChatPerplexity(temperature=0, model=\"llama-3.1-sonar-small-128k-online\")"
+    "chat = ChatPerplexity(temperature=0, model=\"sonar\")"
    ]
   },
   {
@@ -200,7 +198,7 @@
     }
    ],
    "source": [
-    "chat = ChatPerplexity(temperature=0, model=\"llama-3.1-sonar-small-128k-online\")\n",
+    "chat = ChatPerplexity(temperature=0, model=\"sonar\")\n",
     "prompt = ChatPromptTemplate.from_messages([(\"human\", \"Tell me a joke about {topic}\")])\n",
     "chain = prompt | chat\n",
     "response = chain.invoke({\"topic\": \"cats\"})\n",
@@ -235,7 +233,7 @@
     }
    ],
    "source": [
-    "chat = ChatPerplexity(temperature=0.7, model=\"llama-3.1-sonar-small-128k-online\")\n",
+    "chat = ChatPerplexity(temperature=0.7, model=\"sonar\")\n",
     "response = chat.invoke(\n",
     "    \"Tell me a joke about cats\", extra_body={\"search_recency_filter\": \"week\"}\n",
     ")\n",
@@ -284,7 +282,7 @@
     }
    ],
    "source": [
-    "chat = ChatPerplexity(temperature=0.7, model=\"llama-3.1-sonar-small-128k-online\")\n",
+    "chat = ChatPerplexity(temperature=0.7, model=\"sonar\")\n",
     "\n",
     "for chunk in chat.stream(\"Give me a list of famous tourist attractions in Pakistan\"):\n",
     "    print(chunk.content, end=\"\", flush=True)"