Update training.py

pylint compliancy

python-package/xgboost/training.py

@@ -52,7 +52,8 @@ def train(params, dtrain, num_boost_round=10, evals=(), obj=None, feval=None,
     learning_rates: list or function
         List of learning rate for each boosting round
         or a customized function that calculates eta in terms of
-        current number of round and the total number of boosting round (e.g. yields learning rate decay)
+        current number of round and the total number of boosting round (e.g. yields
+        learning rate decay)
         - list l: eta = l[boosting round]
         - function f: eta = f(boosting round, num_boost_round)
     xgb_model : file name of stored xgb model or 'Booster' instance