[DOC] fix syntax error for reference in plot_cost_complexity_pruning.py (scikit-learn#15423)

Author: tirthasheshpatel

examples/tree/plot_cost_complexity_pruning.py

@@ -14,7 +14,7 @@
 ``ccp_alpha`` on regularizing the trees and how to choose a ``ccp_alpha``
 based on validation scores.
 
-See also `ref`:_minimal_cost_complexity_pruning` for details on pruning.
+See also :ref:`minimal_cost_complexity_pruning` for details on pruning.
 """
 
 print(__doc__)