Fix KNN error message. (rapidsai#4782)

Authors:
  - Jiaming Yuan (https://github.com/trivialfis)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: rapidsai#4782

python/cuml/neighbors/nearest_neighbors.pyx

@@ -362,7 +362,7 @@ class NearestNeighbors(Base,
 
         if is_sparse(X):
             valid_metrics = cuml.neighbors.VALID_METRICS_SPARSE
-            value_metric_str = "_SPARSE"
+            valid_metric_str = "_SPARSE"
             self.X_m = SparseCumlArray(X, convert_to_dtype=cp.float32,
                                        convert_format=False)
             self.n_rows = self.X_m.shape[0]