[docs] Fix repetition of `tree_method` hyper-parameter in xgboost_hist experiment (#3296)

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

docs/Experiments.rst

@@ -73,7 +73,6 @@ We set up total 3 settings for experiments. The parameters of these settings are
        eta = 0.1
        num_round = 500
        nthread = 16
-       tree_method = approx
        min_child_weight = 100
        tree_method = hist
        grow_policy = lossguide