docs: Cosmetic documentation fix. Update llm_chain.ipynb. (langchai…

…n-ai#27394)

ATM [LLM chain
docs](https://python.langchain.com/docs/tutorials/llm_chain/#server)
say:

```
# 3. Create parser
parser = StrOutputParser()

# 4. Create chain
chain = prompt_template | model | parser


# 4. App definition
app = FastAPI(
  title="LangChain Server",
  version="1.0",
  description="A simple API server using LangChain's Runnable interfaces",
)

# 5. Adding chain route
add_routes(
    app,
    chain,
    path="/chain",
)
```

I corrected it to:


```
# 3. Create parser
parser = StrOutputParser()

# 4. Create chain
chain = prompt_template | model | parser

# 5. App definition
app = FastAPI(
  title="LangChain Server",
  version="1.0",
  description="A simple API server using LangChain's Runnable interfaces",
)

# 6. Adding chain route
add_routes(
    app,
    chain,
    path="/chain",
)
```

docs/docs/tutorials/llm_chain.ipynb

@@ -497,15 +497,14 @@
     "# 4. Create chain\n",
     "chain = prompt_template | model | parser\n",
     "\n",
-    "\n",
-    "# 4. App definition\n",
+    "# 5. App definition\n",
     "app = FastAPI(\n",
     "  title=\"LangChain Server\",\n",
     "  version=\"1.0\",\n",
     "  description=\"A simple API server using LangChain's Runnable interfaces\",\n",
     ")\n",
     "\n",
-    "# 5. Adding chain route\n",
+    "# 6. Adding chain route\n",
     "add_routes(\n",
     "    app,\n",
     "    chain,\n",