[docs] Add mode to Ray Tune quick start

[Crissman]

Why are these changes needed?

If mode is not specified in analysis.get_best_config, it gives this error:

Traceback (most recent call last):
  File "ray_quick.py", line 25, in <module>
    print("Best config: ", analysis.get_best_config(metric="mean_loss"))
  File "/users/Crissman/anaconda3/lib/python3.7/site-packages/ray/tune/analysis/experiment_analysis.py", line 607, in get_best_config
    best_trial = self.get_best_trial(metric, mode, scope)
  File "/users/Crissman/anaconda3/lib/python3.7/site-packages/ray/tune/analysis/experiment_analysis.py", line 538, in get_best_trial
    mode = self._validate_mode(mode)
  File "/users/Crissman/anaconda3/lib/python3.7/site-packages/ray/tune/analysis/experiment_analysis.py", line 82, in _validate_mode
    "No `mode` has been passed and  `default_mode` has "
ValueError: No `mode` has been passed and  `default_mode` has not been set. Please specify the `mode` parameter.

Related issue number

N/A

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[richardliaw]

thanks a bunch for this catch!