docs: Correct formula in jount-coupling task

Author: stephane-caron

pink/tasks/joint_coupling_task.py

@@ -22,21 +22,21 @@ class JointCouplingTask(LinearHolonomicTask):
 
     .. math::
 
-        \sum_{j \in J} r_j q_j
+        \sum_{j \in J} r_j q_j = 0
 
     where :math:`J` is a given set of joints the task is configured for,
     :math:`r_j` is the "ratio" associated with a joint :math:`j \in J` and
     :math:`q_j` is the current joint angle (in the configuration vector
     :math:`q`).
 
-    A simple use case of this task is:
+    For instance, the humanoid Draco 3 examples uses this task to enforce:
 
     .. math::
 
         q_1 - q_2 = 0
 
-    where the two joints in :math:`J = {1, 2}` are coupled to rotate in
-    opposite directions.
+    where two joints :math:`J = \{1, 2\}` are coupled to share the same joint
+    angle. The ratios are then :math:`r_1 = 1` and :math:`r_2 = -1`.
 
     This task is a particular case of the more general
     :class:`pink.tasks.linear_holonomic_task.LinearHolonomicTask`.