message: Fix broken link pointing to vulnerability reporting/SECURITY.md
  The vulnerability reporting (SECURITY.md) has been moved to top level
  directory, this fix fixes the broken link inside tensorflow/docs_src/community/welcome.md
diff: @@ -65,5 +65,5 @@ please read the following list carefully:
  on GitHub. For example, use the issue tracker to request a
  new operation in TensorFlow.
  * To report vulnerabilities, please follow our
  - [vulnerability disclosure guidelines](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/SECURITY.md).
  + [vulnerability disclosure guidelines](https://github.com/tensorflow/tensorflow/blob/master/SECURITY.md).
  