DOC: Incorrect type for tags parameter in MLflow callback

[ArchimedesFTW]

Issue with current documentation:

In the documentation the tag type is string, but in the code it's a dictionary.

The proposed fix is to change the following two lines "tags (str):" to "tags (dict):".

https://github.com/hwchase17/langchain/blob/7414e9d19603c962063dd337cdcf3c3168d4b8be/langchain/callbacks/mlflow_callback.py#L120

https://github.com/hwchase17/langchain/blob/7414e9d19603c962063dd337cdcf3c3168d4b8be/langchain/callbacks/mlflow_callback.py#L225

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