Docs: Add redline to highlight main process (#3977)

docs/backend/offline_engine_api.ipynb

@@ -18,7 +18,8 @@
     "- Non-streaming asynchronous generation\n",
     "- Streaming asynchronous generation\n",
     "\n",
-    "**To launch the offline engine in your python scripts, `__main__` condition is necessary, since we use `spawn` mode to create subprocesses. Please refer to this [simple example](https://github.com/sgl-project/sglang/blob/main/examples/runtime/engine/launch_engine.py) for more details.**\n",
+    "\n",
+    "<span style=\"color:red\">**To launch the offline engine in your python scripts,** `__main__` **condition is necessary, since we use** `spawn` **mode to create subprocesses. Please refer to this** [simple example](https://github.com/sgl-project/sglang/blob/main/examples/runtime/engine/launch_engine.py) **for more details.**</span>\n",
     "\n",
     "Additionally, you can easily build a custom server on top of the SGLang offline engine. A detailed example working in a python script can be found in [custom_server](https://github.com/sgl-project/sglang/blob/main/examples/runtime/engine/custom_server.py)."
    ]