DOC add a docstring example for the learning_curve function (#24546)
DOC add a docstring example for the learning_curve function (#24546)

Co-authored-by: Thomas J. Fan <5e8796a332fafc9c261437cd1787ae2709a896a7@gmail.com>
vinayak-mehta/scikit-learn,betatim/scikit-learn,lesteve/scikit-learn,espg/scikit-learn,espg/scikit-learn,lesteve/scikit-learn,vinayak-mehta/scikit-learn,espg/scikit-learn,betatim/scikit-learn,espg/scikit-learn,lesteve/scikit-learn,betatim/scikit-learn,vinayak-mehta/scikit-learn,scikit-learn/scikit-learn,vinayak-mehta/scikit-learn,scikit-learn/scikit-learn,betatim/scikit-learn,scikit-learn/scikit-learn,lesteve/scikit-learn,scikit-learn/scikit-learn