servo tutorial: stick to version 1.2.7 of UR repo (moveit#750)

Stick to older version of Universal Robot Github (1.2.7 ) as newer versions (1.3.x) are incompatible with the servo tutorial.
Co-authored-by: Robert Haschke <rhaschke@users.noreply.github.com>

Author: Akumar201

doc/realtime_servo/realtime_servo_tutorial.rst

@@ -22,7 +22,7 @@ Clone `universal_robot melodic-devel branch <https://github.com/ros-industrial/u
 
     cd ~/ws_moveit/src
 
-    git clone -b melodic-devel https://github.com/ros-industrial/universal_robot
+    git clone -b 1.2.7 https://github.com/ros-industrial/universal_robot
 
 Install any new dependencies that may be missing: ::