doc: fix async reducer README examples

[th0ger]

Fix #243

Tested with pynumaflow 0.9.2.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.71%. Comparing base (17e063b) to head (ca954f1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
- Coverage   94.35%   93.71%   -0.64%     
==========================================
  Files          65       65              
  Lines        2868     2866       -2     
  Branches      150      150              
==========================================
- Hits         2706     2686      -20     
+ Misses        131      130       -1     
- Partials       31       50      +19     

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