Update Example_Batch.md

[Silverados]

message.getBody() return byte[], so change to length.

Trivial changes~

Make sure set the target branch to develop

What is the purpose of the change

Brief changelog

Verifying this change

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[codecov-commenter]

Codecov Report

Merging #3304 (4bfa004) into develop (fb8bc64) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop    #3304      +/-   ##
=============================================
- Coverage      47.98%   47.97%   -0.02%     
+ Complexity      4562     4334     -228     
=============================================
  Files            552      514      -38     
  Lines          36521    34989    -1532     
  Branches        4818     4665     -153     
=============================================
- Hits           17525    16785     -740     
+ Misses         16781    16019     -762     
+ Partials        2215     2185      -30     

Impacted Files	Coverage Δ
...org/apache/rocketmq/store/ha/WaitNotifyObject.java	66.07% <0.00%> (-5.36%)	⬇️
...ketmq/client/impl/consumer/PullMessageService.java	71.11% <0.00%> (-4.45%)	⬇️
...a/org/apache/rocketmq/filter/util/BloomFilter.java	59.13% <0.00%> (-2.16%)	⬇️
.../rocketmq/broker/filter/ConsumerFilterManager.java	72.19% <0.00%> (-1.80%)	⬇️
...he/rocketmq/client/trace/AsyncTraceDispatcher.java	77.72% <0.00%> (-1.49%)	⬇️
...a/org/apache/rocketmq/store/StoreStatsService.java	29.50% <0.00%> (-0.66%)	⬇️
...e/rocketmq/client/impl/consumer/RebalanceImpl.java	40.23% <0.00%> (-0.40%)	⬇️
...n/java/org/apache/rocketmq/store/ha/HAService.java	70.95% <0.00%> (-0.34%)	⬇️
...mq/client/impl/producer/DefaultMQProducerImpl.java	45.11% <0.00%> (-0.13%)	⬇️
...ava/org/apache/rocketmq/filter/util/BitsArray.java	59.82% <0.00%> (ø)
... and 46 more

---

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[coveralls]

Coverage decreased (-0.07%) to 54.154% when pulling 4bfa004 on Silverados:patch-2 into 2cac866 on apache:develop.

[yuz10]

would you change 2 other places in the document?