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

@@ -70,7 +70,7 @@ The critical load :math:`(\tau_{xy})_{cr}` of a stiffened panel subject to shear
 .. math::
   (\tau_{xy})_{cr}=k_{xy}\frac{\pi^2D}{b_0^2t_s}
 
-where
+where:
 
 - :math:`a` is the width of the panel;