changed to mlflow-ai-gateway in llms/__init__.py (#8114)

- Description: In the llms/__init__.py, the key name is wrong for
mlflowaigateway. It should be mlflow-ai-gateway
  - Issue: NA
  - Dependencies: NA
  - Tag maintainer: @hwchase17, @baskaryan
  - Twitter handle: na

Without this fix, when we run the code for mlflowaigateway, we will get
error as below

ValueError: Loading mlflow-ai-gateway LLM not supported

---------

Co-authored-by: rajib76 <rajib76@yahoo.com>
Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/llms/__init__.py

@@ -150,7 +150,7 @@ type_to_cls_dict: Dict[str, Type[BaseLLM]] = {
     "koboldai": KoboldApiLLM,
     "llamacpp": LlamaCpp,
     "textgen": TextGen,
-    "mlflow-gateway": MlflowAIGateway,
+    "mlflow-ai-gateway": MlflowAIGateway,
     "modal": Modal,
     "mosaic": MosaicML,
     "nlpcloud": NLPCloud,