DefaultCrytoMaterialsManager docstring incorrectly indicates that algorithm is configurable. Behavior is as intended, but I missed clearing out this docstring.

https://github.com/awslabs/aws-encryption-sdk-python/blob/b6a2bbca4db6ab46d65fe7e2eacfffe25abff3a2/aws_encryption_sdk/materials_managers/default.py#L26