add default value of gamma in parameter.md

doc/parameter.md

@@ -27,7 +27,7 @@ From xgboost-unity, the ```bst:``` prefix is no longer needed for booster parame
 #### Parameter for Tree Booster
 * eta [default=0.3]
   - step size shrinkage used in update to prevents overfitting. After each boosting step, we can directly get the weights of new features. and eta actually shrinkage the feature weights to make the boosting process more conservative.
-* gamma 
+* gamma [default=0]
   - minimum loss reduction required to make a further partition on a leaf node of the tree. the larger, the more conservative the algorithm will be.
 * max_depth [default=6]
   - maximum depth of a tree