DOC fix deprecation warning in plot_lof_novelty_detection (scikit-learn#27451)

Author: glemaitre

examples/neighbors/plot_lof_novelty_detection.py

@@ -26,6 +26,7 @@
 """
 
 import matplotlib
+import matplotlib.lines as mlines
 import matplotlib.pyplot as plt
 import numpy as np
 
@@ -71,7 +72,7 @@
 plt.xlim((-5, 5))
 plt.ylim((-5, 5))
 plt.legend(
-    [a.collections[0], b1, b2, c],
+    [mlines.Line2D([], [], color="darkred"), b1, b2, c],
     [
         "learned frontier",
         "training observations",