doc: minor corrections in pH tutorial

[franckgaga]

preview: https://juliacontrol.github.io/ModelPredictiveControl.jl/previews/PR472

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.23%. Comparing base (ae59d13) to head (d6d0be1).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #472      +/-   ##
==========================================
+ Coverage   97.20%   97.23%   +0.02%     
==========================================
  Files          28       28              
  Lines        6985     6985              
==========================================
+ Hits         6790     6792       +2     
+ Misses        195      193       -2     

Flag	Coverage Δ
unittests	97.23% <ø> (+0.02%)	⬆️

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