DOC avoid the plot_ prefix in example where no plot

Also fix line reference

Author: jnothman

doc/tutorial/statistical_inference/supervised_learning.rst

@@ -400,10 +400,10 @@ This is known as :class:`LogisticRegression`.
    model. Leave out the last 10% and test prediction performance on these
    observations.
 
-   .. literalinclude:: ../../auto_examples/exercises/plot_digits_classification_exercise.py
-       :lines: 12-17
+   .. literalinclude:: ../../auto_examples/exercises/digits_classification_exercise.py
+       :lines: 15-19
 
-   Solution: :download:`../../auto_examples/exercises/plot_digits_classification_exercise.py`
+   Solution: :download:`../../auto_examples/exercises/digits_classification_exercise.py`
 
 
 Support vector machines (SVMs)