Clarify file code snippets are in docs about Kedro Mlflow

[merelcht]

Description

In section https://docs.kedro.org/en/stable/integrations-and-plugins/mlflow/#artifact-tracking-in-mlflow-using-kedro-mlflow, mention clearly that the code snippets are inside conf/base/catalog.yml

Documentation page (if applicable)

https://docs.kedro.org/en/stable/integrations-and-plugins/mlflow/

Contribution guidelines

https://github.com/kedro-org/kedro/wiki/Contribute-to-the-Kedro-documentation