fix typo

Author: charlesdong1991

databricks/koalas/tests/test_series_plot.py

@@ -97,7 +97,7 @@ def test_pie_plot(self):
         self.compare_plots(ax1, ax2)
 
         ax1 = pdf['a'].plot(kind='pie', colormap='Paired')
-        ax2 = kdf['a'].pie(kind='pie', colormap='Paired')
+        ax2 = kdf['a'].plot(kind='pie', colormap='Paired')
         self.compare_plots(ax1, ax2)
 
     def test_pie_plot_limited(self):