Fix CLI ranking demo. (#6439)

Save model at final round.

demo/rank/mq2008.conf

@@ -5,9 +5,9 @@ objective="rank:pairwise"
 
 # Tree Booster Parameters
 # step size shrinkage
-eta = 0.1 
+eta = 0.1
 # minimum loss reduction required to make a further partition
-gamma = 1.0 
+gamma = 1.0
 # minimum sum of instance weight(hessian) needed in a child
 min_child_weight = 0.1
 # maximum depth of a tree
@@ -17,12 +17,10 @@ max_depth = 6
 # the number of round to do boosting
 num_round = 4
 # 0 means do not save any model except the final round model
-save_period = 0 
+save_period = 0
 # The path of training data
-data = "mq2008.train" 
+data = "mq2008.train"
 # The path of validation data, used to monitor training process, here [test] sets name of the validation set
-eval[test] = "mq2008.vali" 
-# The path of test data 
-test:data = "mq2008.test"      
-
-
+eval[test] = "mq2008.vali"
+# The path of test data
+test:data = "mq2008.test"