avoid returning self at in py script ch02

Author: rasbt

code/optional-py-scripts/ch02.py

@@ -410,4 +410,4 @@ def predict(self, X):
 # plt.savefig('./adaline_5.png', dpi=300)
 plt.show()
 
-ada.partial_fit(X_std[0, :], y[0])
+ada = ada.partial_fit(X_std[0, :], y[0])