differential_inverse_kinematics: Fix docstring formatting (RobotLocom…

…otion#13892)

* differential_inverse_kinematics: Fix docstring formatting

manipulation/planner/differential_inverse_kinematics.h

@@ -230,6 +230,7 @@ class DifferentialInverseKinematicsParameters {
  * Computes a generalized velocity v_next, via the following
  * MathematicalProgram:
  *
+ * ```
  *   min_{v_next,alpha}   100 * | alpha - |V| |^2
  *                        // iff J.rows() < J.cols(), then
  *                          + | q_current + v_next*dt - q_nominal |^2
@@ -253,6 +254,7 @@ class DifferentialInverseKinematicsParameters {
  *   can have any size, with each element representing a constraint on the
  *   solution (6 constraints specifying an end-effector pose is typical, but
  *   not required).
+ * ```
  *
  * Intuitively, this finds a v_next such that J*v_next is in the same direction
  * as V, and the difference between |V| and |J * v_next| is minimized while all