[Doc] Edited minor typo (#26266)

Signed-off-by: Orange Ng <ngquanhao@outlook.com>

docs/design/plugin_system.md

@@ -49,7 +49,7 @@ Every plugin has three parts:
 
 - **Platform plugins** (with group name `vllm.platform_plugins`): The primary use case for these plugins is to register custom, out-of-the-tree platforms into vLLM. The plugin function should return `None` when the platform is not supported in the current environment, or the platform class's fully qualified name when the platform is supported.
 
-- **IO Processor plugins** (with group name `vllm.io_processor_plugins`): The primary use case for these plugins is to register custom pre/post processing of the model prompt and model output for poling models. The plugin function returns the IOProcessor's class fully qualified name.
+- **IO Processor plugins** (with group name `vllm.io_processor_plugins`): The primary use case for these plugins is to register custom pre/post processing of the model prompt and model output for pooling models. The plugin function returns the IOProcessor's class fully qualified name.
 
 ## Guidelines for Writing Plugins