doc: correction in pH equation

[franckgaga]

There is no 10 factor in front of the $\log_{10}$:

$$ \mathrm{pH} = - \log_{10}(a_H) $$

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.17%. Comparing base (455a01e) to head (62686c6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #461      +/-   ##
==========================================
- Coverage   97.20%   97.17%   -0.03%     
==========================================
  Files          28       28              
  Lines        6867     6867              
==========================================
- Hits         6675     6673       -2     
- Misses        192      194       +2     

Flag	Coverage Δ
unittests	97.17% <ø> (-0.03%)	⬇️

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