message: Fix code highlight language in profiler README
diff: @@ -54,7 +54,7 @@ with tf.contrib.tfprof.ProfileContext() as pctx:
  train_loop()
  ```
  -```python
  +```shell
  # Profiling from Python API is not interactive.
  # Dump the profiles to files and profile with interactive command line.
  with tf.contrib.tfprof.ProfileContext() as pctx:
  @@ -137,7 +137,7 @@ ApplyAdam 231.65MB (85.28%, 0.31%), 92.66ms (23.43%,
  ### Auto-profile.
  -```
  +```shell
  tfprof> advise
  Not running under xxxx. Skip JobChecker.
  @@ -194,8 +194,9 @@ seq2seq_attention_model.py:363:build_graph:self._add_train_o..., cpu: 1.28sec, a
  optimizer.py:97:update_op:return optimizer...., cpu: 84.76ms, accelerator: 0us, total: 84.76ms
  ```
  -### Visualize time and memory.
  -```
  +### Visualize time and memory
  +
  +```shell
  # The following example generates a timeline.
  tfprof> graph -step 0 -max_depth 100000 -output timeline:outfile=<filename>
  @@ -209,7 +210,7 @@ Open a Chrome browser, enter URL chrome://tracing and load the timeline file.
  ![Timeline](g3doc/graph_timeline.png)
  -```
  +```shell
  # The following example generates a pprof graph (only supported by code view).
  # Since TensorFlow runs the graph instead of Python code, the pprof graph
  # doesn't profile the statistics of Python, but the TensorFlow graph
  