CONTRIBUTING.md Docker section newlines

Author: ahundt

CONTRIBUTING.md

@@ -160,10 +160,12 @@ There are two ways to run TensorFlow unit tests.
    ```
 
 2. Using [Docker](www.docker.com) and TensorFlow's CI scripts.
+
    ```bash
    # Install Docker first, then this will build and run cpu tests
    tensorflow/tools/ci_build/ci_build.sh CPU bazel test //tensorflow/...
    ```
+
    See
    [TensorFlow Builds](https://github.com/tensorflow/tensorflow/tree/master/tensorflow/tools/ci_build) for details.