Update python/doc/usecases/use_case_stiffened_panel.rst

Co-authored-by: Michael Baudin <31351465+mbaudin47@users.noreply.github.com>

python/doc/usecases/use_case_stiffened_panel.rst

@@ -137,7 +137,7 @@ where:
 - :math:`\ell` is the length of the stiffener flank;
 
 - :math:`d=\frac{\ell-f_2}{2}-R\theta- :math:` is the half-lenght of the straight
-  part of the flank of the stiffener;
+  part of the side of the stiffener;
 
 - :math:`A=\ell t_c` is the area of the section of an half-ondulation;