[python-package] f-string format updated in plot_example.py (#4421)

* f-string format updated.

* space removed

* Update examples/python-guide/plot_example.py according to suggestion.
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

examples/python-guide/plot_example.py

@@ -37,7 +37,7 @@ gbm = lgb.train(params,
                 lgb_train,
                 num_boost_round=100,
                 valid_sets=[lgb_train, lgb_test],
-                feature_name=['f' + str(i + 1) for i in range(X_train.shape[-1])],
+                feature_name=[f'f{i + 1}' for i in range(X_train.shape[-1])],
                 categorical_feature=[21],
                 evals_result=evals_result,
                 verbose_eval=10)