Replace llm variable with model (#23280)

The code snippet under ‘pdfs_qa’ contains an small incorrect code
example , resulting in users getting errors. This pr replaces ‘llm’
variable with ‘model’ to help user avoid a NameError message.

Resolves #22689


If no one reviews your PR within a few days, please @-mention one of
baskaryan, efriis, eyurtsev, ccurme, vbarda, hwchase17.

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

docs/docs/tutorials/pdf_qa.ipynb

@@ -122,7 +122,7 @@
     "```{=mdx}\n",
     "import ChatModelTabs from \"@theme/ChatModelTabs\";\n",
     "\n",
-    "<ChatModelTabs openaiParams={`model=\"gpt-4o\"`} />\n",
+    "<ChatModelTabs customVarName=\"llm\" openaiParams={`model=\"gpt-4o\"`} />\n",
     "```"
    ]
   },