DOC fix warning message in plot_narx_msa

[MatthewSZhang]

Checklist

• Used a personal fork to propose changes
• A reference to a related issue: DOC deal with warning message in plot_narx_msa due to optimizer changed from least_sqaures to minimize #61
• A description of the changes

Specify Nelder-Mead method when coef_init="one_step_ahead". The method is slower, but gives better accuracy and no warning message.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b389610) to head (413baa2).
Report is 1 commits behind head on main.

Additional details and impacted files

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##              main       #62   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            6         6           
  Lines          476       476           
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  Hits           476       476           

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