fix error dual objective sensitivity mention

Author: andrewrosemberg

docs/src/usage.md

@@ -188,7 +188,7 @@ It is important to note that the (reverse) parameter perturbation given an objec
 
 **Dual Objective Sensitivity**
 
-In addition to the primal objective sensitivity, one could also calculate the dual objective sensitivity with respect to the parameters. The dual objective sensitivity is calculated using the optimal solution of the dual problem and the sensitivities of the dual variables with respect to the parameters.
+In addition to the primal objective sensitivity, one could also calculate the dual objective sensitivity with respect to the parameters using the gradients of the dual objective and the sensitivities of the dual variables with respect to the parameters.
 This is currently not implemented for any problem class, but will be available in future releases.
 
 Note that the dual objective sensitivity is equivalent to the primal objective sensitivity problems where strong duality holds.