[examples] updated tree index with categorical feature (#2044)

* updated gitignore

* updated tree index with cat feature

.gitignore

@@ -393,3 +393,7 @@ lightgbm.Rcheck/
 
 # Files generated by aspell
 **/*.bak
+
+# GraphViz artifacts
+*.gv
+*.gv.*

examples/python-guide/plot_example.py

@@ -50,10 +50,10 @@ print('Plotting feature importances...')
 ax = lgb.plot_importance(gbm, max_num_features=10)
 plt.show()
 
-print('Plotting 84th tree...')  # one tree use categorical feature to split
-ax = lgb.plot_tree(gbm, tree_index=83, figsize=(20, 8), show_info=['split_gain'])
+print('Plotting 54th tree...')  # one tree use categorical feature to split
+ax = lgb.plot_tree(gbm, tree_index=53, figsize=(15, 15), show_info=['split_gain'])
 plt.show()
 
-print('Plotting 84th tree with graphviz...')
-graph = lgb.create_tree_digraph(gbm, tree_index=83, name='Tree84')
+print('Plotting 54th tree with graphviz...')
+graph = lgb.create_tree_digraph(gbm, tree_index=53, name='Tree54')
 graph.render(view=True)