update branch for realtime_tools (#1200)

realtime_tools branched out for humble, so we should update that in our repos file.

Author: urfeex

Universal_Robots_ROS2_Driver.humble.repos

@@ -30,4 +30,4 @@ repositories:
   realtime_tools:
     type: git
     url: https://github.com/ros-controls/realtime_tools.git
-    version: master
+    version: humble