!3552 transformer数据预处理部分readme修改，增加易用性

Merge pull request !3552 from jijiarong/master

official/nlp/transformer/README.md

@@ -345,7 +345,7 @@ Parameters for learning rate:
 
 - Convert the original data to mindrecord for evaluation:
 
-    - The 'bucket' parameter is configured through the yaml file
+    - The 'bucket' parameter is configured as [128] through the yaml file
 
     ``` bash
     paste newstest2014.tok.bpe.32000.en newstest2014.tok.bpe.32000.de > test.all

official/nlp/transformer/README_CN.md

@@ -347,7 +347,7 @@ Parameters for learning rate:
 
 - 将原数据转化为MindRecord数据格式进行评估：
 
-    - 'bucket' 参数通过yaml文件配置
+    - 'bucket' 参数通过yaml文件配置为[128]
 
     ``` bash
     paste newstest2014.tok.bpe.32000.en newstest2014.tok.bpe.32000.de > test.all