doc: clearer J prediction matrices for MHE

[franckgaga]

The pattern for the prediction matrix $\mathbf{J}$ was not clear enough for the MHE with $p=0$. It needs one additional column, similarly to the matrix for $p=1$.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.72%. Comparing base (e7ef3db) to head (ea664c2).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #406      +/-   ##
==========================================
- Coverage   98.74%   98.72%   -0.02%     
==========================================
  Files          27       27              
  Lines        5722     5722              
==========================================
- Hits         5650     5649       -1     
- Misses         72       73       +1     

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