Fix ref for source build

Author: christophfroehlich

.github/workflows/rolling-source-build.yml

@@ -23,7 +23,7 @@ jobs:
           OS_NAME: ubuntu-latest
     with:
       ros_distro: ${{ matrix.ROS_DISTRO }}
-      ref: ros2-master
+      ref: master
       ros2_repo_branch: ${{ matrix.ROS_DISTRO }}
       os_name: ${{ matrix.OS_NAME }}
       container: ${{ matrix.CONTAINER }}