Update docker_build_instructions.md

Author: peteflorence

doc/docker_build_instructions.md

@@ -36,7 +36,6 @@ and then restart your machine.
 ```
 git clone [email protected]:RobotLocomotion/pytorch-dense-correspondence.git
 cd pytorch-dense-correspondence
-git submodule sync --recursive
 git submodule update --init --recursive
 cd docker
 ./docker_build.py