Fix joystick velocity teleop deadman button

A bug in how the deadman button was set prevented users from being able to send Twist commands to the cmd_vel topic using the Joystick controller in the teleop_twist.launch file.

stretch_core/launch/teleop_twist.launch

@@ -23,7 +23,7 @@
         <param name="deadzone" value="0.05" />
     </node>
     <node name="teleop_twist_joy" pkg="teleop_twist_joy" type="teleop_node">
-        <param name="enable_button" type="int" value="-1" /> <!-- value="0" to enable deadman button -->
+        <param name="enable_button" type="int" value="0" /> <!-- The 'A' button (green one) on the controller is the deadman button -->
         <param name="scale_linear" type="double" value="$(arg linear)" />
         <param name="scale_angular" type="double" value="$(arg angular)" />
         <remap from="/cmd_vel" to="$(arg twist_topic)" />