doc: argument order bugfix in pH tutorial

[franckgaga]

I reversed the order of the ym and u arguments in updatestate! method for the MHE. There is no longer an unexpected offset between $\mathbf{y}$ and $\mathbf{\hat{y}}$ for $20 \le t \le 30$ h on the pH neutralization tank.

Gotta say, I'm surprised that it was still working with the arguments in the reversed order, but it's only a stroke of luck (or bad luck?) 😆

About this, it's kind of impossible to catch this and throw a clear error message to the user. If it happens that the input and measured output vectors have the same length like here, it's impossible to detect a user mistake like this. That's the major drawback of a "non-symbolic" toolkit like this package.

Here's the preview : https://juliacontrol.github.io/ModelPredictiveControl.jl/previews/PR459

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.21%. Comparing base (7fa412c) to head (618e6c8).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
+ Coverage   97.17%   97.21%   +0.04%     
==========================================
  Files          28       28              
  Lines        6867     6867              
==========================================
+ Hits         6673     6676       +3     
+ Misses        194      191       -3     

Flag	Coverage Δ
unittests	97.21% <ø> (+0.04%)	⬆️

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