DOC fix indent in docstring of AdditiveChi2sampler (scikit-learn#20710)

Co-authored-by: Guillaume Lemaitre <[email protected]>

Author: reshamas

sklearn/kernel_approximation.py

@@ -552,7 +552,7 @@ def fit(self, X, y=None):
             and `n_features` is the number of features.
 
         y : array-like, shape (n_samples,) or (n_samples, n_outputs), \
-            default=None
+                default=None
             Target values (None for unsupervised transformations).
 
         Returns