fixed parameter name problems -> problem

[neil-rubens]

using parameter problem instead of problemes fixes error:
p.s. see discussion https://gitter.im/tensor2tensor/Lobby?at=5b0273bdf04ce53632f379ce

 File "/root/.local/lib/python2.7/site-packages/tensor2tensor/utils/registry.py", line 251, in parse_problem_name
    if problem_name.endswith("_rev"):
AttributeError: 'NoneType' object has no attribute 'endswith'

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[neil-rubens]

I signed it.

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