fix typo (idomatic -> idiomatic)

[Maisy]

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• [x] I have read the CONTRIBUTING guidelines.
• [x] I have added test cases which verify the correct operation of this feature or patch.
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[coveralls]

Coverage remained the same at 99.79% when pulling 74b34cfcbfbb37c163e0ef5c5b6e8a3f071d327c on Maisy:master into 1105760de472c25e2641b8c2b4058189556fb2e2 on jaredhanson:master.