[sgd] hotfix example failure

[richardliaw]

What do these changes do?

ccf24c74f16db424ba024ffd887c3b6d2428d5f0b7e54f29c439321d52b9a13a python /ray/python/ray/experimental/sgd/examples/train_example.py

real	0m2.553s
user	0m0.048s
sys	0m0.028s
WARNING: Your kernel does not support swap limit capabilities or the cgroup is not mounted. Memory limited without swap.
usage: train_example.py [-h] --redis-address REDIS_ADDRESS [--use-gpu]
train_example.py: error: the following arguments are required: --redis-address
FAILED 2

Fixes the Jenkins test.

Related issue number

Linter

• I've run scripts/format.sh to lint the changes in this PR.

[AmplabJenkins]

Test FAILed.
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Test FAILed.

[AmplabJenkins]

Test PASSed.
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Test PASSed.