Spelling Error (_medical_likeliehood)

[jnhasty]

_medical_likeliehood -> _medical_likelihood

https://github.com/GoogleCloudPlatform/google-cloud-python/blob/b28a4eb667ae08c3f4dcf9af891ed4931884989c/vision/google/cloud/vision/safe_search.py#L43

[tseaver]

@jnhasty Thank you for the report!