[GEARPUMP-292] Update wordcount example in the doc

[manuzhang]

Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

• Make sure the commit message is formatted like:
  [GEARPUMP-<Jira issue #>] Meaningful description of pull request
• Make sure tests pass via sbt clean test.
• Make sure old documentation affected by the pull request has been updated and new documentation added for new functionality.

[codecov-io]

Codecov Report

Merging #171 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #171      +/-   ##
==========================================
- Coverage   71.63%   71.62%   -0.02%     
==========================================
  Files         191      191              
  Lines        6086     6086              
  Branches      527      527              
==========================================
- Hits         4360     4359       -1     
- Misses       1726     1727       +1

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[huafengw]

+1