message: Fix build doc README
  `src_dir` must be absolute path
  docs_src was moved to `tensorflow/docs_src`
diff: @@ -5,7 +5,7 @@ go to:
  Documentation (on Github, tensorflow.org, and anywhere else we decide to
  serve it from) is now generated from the files in
  -third_party/tensorflow/docs_src/ (for tutorials and other guides) and
  +tensorflow/docs_src/ (for tutorials and other guides) and
  TensorFlow source code (for the API reference pages). If you see a problem with
  API reference, edit the code comments in the appropriate language. If you see a
  problem with our other docs, edit the files in docs_src.
  @@ -14,9 +14,10 @@ To preview the results of your changes, or generate an offline copy of
  the docs, run:
  bazel run -- tensorflow/tools/docs:generate \
  - --src_dir=tensorflow/docs_src/ \
  + --src_dir=/path/to/tensorflow/docs_src/ \
  --output_dir=/tmp/tfdocs/
  +`src_dir` must be absolute path to documentation source.
  When authoring docs, note that we have some new syntax for references --
  at least for docs coming from Python docstrings or
  tensorflow/docs_src/. Use:
  @@ -26,7 +27,6 @@ tensorflow/docs_src/. Use:
  syntax still works, since @{tf.MyClass.method} links to the right
  part of the tf.MyClass page.
  -
  * @{tensorflow::symbol} to make a link to the reference page for a C++
  symbol. (This only works for a few symbols but will work for more soon.)
  