Fix typo in arg_write docstring

[Sbozzolo]

As title

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.54%. Comparing base (997089b) to head (158676d).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
+ Coverage   87.50%   88.54%   +1.04%     
==========================================
  Files           1        1              
  Lines          96       96              
==========================================
+ Hits           84       85       +1     
+ Misses         12       11       -1     

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[DilumAluthge]

Thank you @Sbozzolo!