documentation: fixing typo (scikit-learn#6938)

the documentation of the form of regular and nested parameters in set_params is corrected

Author: jstriebel

sklearn/base.py

@@ -253,7 +253,7 @@ def set_params(self, **params):
         """Set the parameters of this estimator.
 
         The method works on simple estimators as well as on nested objects
-        (such as pipelines). The former have parameters of the form
+        (such as pipelines). The latter have parameters of the form
         ``<component>__<parameter>`` so that it's possible to update each
         component of a nested object.