DOC: Fixed warning in doc build (#21449)

Author: TomAugspurger

doc/source/ecosystem.rst

@@ -39,7 +39,7 @@ Use pandas DataFrames in your `scikit-learn <http://scikit-learn.org/>`__
 ML pipeline.
 
 `Featuretools <https://github.com/featuretools/featuretools/>`__
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+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Featuretools is a Python library for automated feature engineering built on top of pandas. It excels at transforming temporal and relational datasets into feature matrices for machine learning using reusable feature engineering "primitives". Users can contribute their own primitives in Python and share them with the rest of the community.