assert: align argument names
This makes sure the documented argument names and the ones thrown in errors is aligned with the actual argument name.
This addresses the comment from @trott https://github.com/nodejs/node/pull/22692#issuecomment-419596446.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines