doc: minor precision in strictly proper system assumption

[franckgaga]

No description provided.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.45%. Comparing base (9c7d276) to head (5715b0d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #263   +/-   ##
=======================================
  Coverage   98.45%   98.45%           
=======================================
  Files          26       26           
  Lines        4464     4464           
=======================================
  Hits         4395     4395           
  Misses         69       69           

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