Replaced instance of affinity with metric (scikit-learn#29608)

Author: decorouz

sklearn/cluster/_agglomerative.py

@@ -999,7 +999,7 @@ def _fit(self, X):
         ----------
         X : ndarray of shape (n_samples, n_features) or (n_samples, n_samples)
             Training instances to cluster, or distances between instances if
-            ``affinity='precomputed'``.
+            ``metric='precomputed'``.
 
         Returns
         -------