Update Understanding-ROS2-Topics.rst (#4843) (#4844)

Missing underscore

Signed-off-by: Mattia Massarenti <121773111+M61ss@users.noreply.github.com>
Co-authored-by: Mattia Massarenti <121773111+M61ss@users.noreply.github.com>
(cherry picked from commit 861a9cc)

source/Tutorials/Beginner-CLI-Tools/Understanding-ROS2-Topics/Understanding-ROS2-Topics.rst

@@ -196,7 +196,7 @@ Recall that the ``cmd_vel`` topic has the type:
 
 This means that in the package ``geometry_msgs`` there is a ``msg`` called ``Twist``.
 
-Now we can run ``ros2 interface show <msg type>`` on this type to learn its details.
+Now we can run ``ros2 interface show <msg_type>`` on this type to learn its details.
 Specifically, what structure of data the message expects.
 
 .. code-block:: console