update gbdt docs (#1720)

docs/en_US/TrialExample/GbdtExample.md

@@ -44,7 +44,15 @@ Given the features and label in train data, we train a GBDT regression model and
 
 ## 3. How to run in nni
 
-### 3.1 Prepare your trial code
+
+### 3.1 Install all the requirments
+
+```
+pip install lightgbm
+pip install pandas
+```
+
+### 3.2 Prepare your trial code
 
 You need to prepare a basic code as following:
 
@@ -86,7 +94,7 @@ if __name__ == '__main__':
     run(lgb_train, lgb_eval, PARAMS, X_test, y_test)
 ```
 
-### 3.2 Prepare your search space.
+### 3.3 Prepare your search space.
 If you like to tune `num_leaves`, `learning_rate`, `bagging_fraction` and `bagging_freq`, you could write a [search_space.json](https://github.com/Microsoft/nni/blob/master/examples/trials/auto-gbdt/search_space.json) as follow:
 
 ```json
@@ -100,7 +108,7 @@ If you like to tune `num_leaves`, `learning_rate`, `bagging_fraction` and `baggi
 
 More support variable type you could reference [here](../Tutorial/SearchSpaceSpec.md).
 
-### 3.3 Add SDK of nni into your code.
+### 3.4 Add SDK of nni into your code.
 
 ```diff
 +import nni
@@ -146,7 +154,7 @@ if __name__ == '__main__':
     run(lgb_train, lgb_eval, PARAMS, X_test, y_test)
 ```
 
-### 3.4 Write a config file and run it.
+### 3.5 Write a config file and run it.
 
 In the config file, you could set some settings including: