[ISSUE #6789] modify placeholder in DefaultMQProducerImpl

[DL1231]

Which Issue(s) This PR Fixes

Fixes #6789

Brief Description

Modify placeholder in DefaultMQProducerImpl.

How Did You Test This Change?

1. Start a producer
2. Produce message synchronously

[codecov-commenter]

Codecov Report

Merging #6790 (cef3050) into develop (bd7db7e) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@              Coverage Diff              @@
##             develop    #6790      +/-   ##
=============================================
- Coverage      42.92%   42.86%   -0.07%     
+ Complexity      8985     8973      -12     
=============================================
  Files           1104     1104              
  Lines          78376    78376              
  Branches       10203    10203              
=============================================
- Hits           33645    33593      -52     
- Misses         40512    40558      +46     
- Partials        4219     4225       +6     

Impacted Files	Coverage Δ
...mq/client/impl/producer/DefaultMQProducerImpl.java	45.84% <0.00%> (ø)

... and 14 files with indirect coverage changes

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