Revert branch typo

Dockerfile.full_ros_build

@@ -5,7 +5,7 @@
 # export CMAKE_BUILD_TYPE=Debug
 #
 # This determines which version of the ROS2 code base to pull
-# export FROM_IMAGE=master
+# export ROS2_BRANCH=master
 #
 # This determines which ros2 docker image to use as a base for getting ros2_dependencies
 # The latest (osrf/ros2:nightly) is usually the best choice unless a dependency
@@ -36,8 +36,8 @@ WORKDIR $ROS2_WS
 
 # clone ROS 2 packages
 
-ARG FROM_IMAGE=master
-RUN wget https://raw.githubusercontent.com/ros2/ros2/$FROM_IMAGE/ros2.repos
+ARG ROS2_BRANCH=master
+RUN wget https://raw.githubusercontent.com/ros2/ros2/$ROS2_BRANCH/ros2.repos
 RUN vcs import src < ros2.repos
 
 # copy nav 2 source code