message: Fix broken link in CONTRIBUTING.md
  This fix fixes the broken link in CONTRIBUTING.md.
  Without `https://`, the markdown will render the link incorrectly to
diff: @@ -163,7 +163,7 @@ There are two ways to run TensorFlow unit tests.
  bazel test ${flags} //tensorflow/python/...
  ```
  -2. Using [Docker](www.docker.com) and TensorFlow's CI scripts.
  +2. Using [Docker](https://www.docker.com) and TensorFlow's CI scripts.
  ```bash
  # Install Docker first, then this will build and run cpu tests
  