fix rwkv.mdx

docs/docs/integrations/llms/rwkv.mdx

@@ -39,6 +39,7 @@ including streaming and CUDA support.
 To use the RWKV wrapper, you need to provide the path to the pre-trained model file and the tokenizer's configuration.
 ```python
 from langchain_community.llms import RWKV
+```
 
 # Test the model