Fix minor typos (#2842)

* Some minor changes to the code style

Some minor changes to the code style in file basic_walkthrough.py

* coding style changes

* coding style changes arrcording PEP8

* Update basic_walkthrough.py

* Fix minor typo

* Minor edits to coding style

Minor edits to coding style following the proposals of PEP8.

demo/multiclass_classification/train.py

@@ -7,7 +7,7 @@ import xgboost as xgb
 
 # label need to be 0 to num_class -1
 data = np.loadtxt('./dermatology.data', delimiter=',',
-        converters={33: lambda x:int(x == '?'), 34: lambda x:int(x)-1})
+        converters={33: lambda x:int(x == '?'), 34: lambda x:int(x) - 1})
 sz = data.shape
 
 train = data[:int(sz[0] * 0.7), :]