_stbt.core.new_device_under_test_from_config: Fix typo
_stbt.core.new_device_under_test_from_config: Fix typo

This didn't affect anything because we never call
`new_device_under_test_from_config` with `transformation_pipeline=None`.
But it's confusing, so it's worth fixing.

Introduced in v23 in commit d173e43 (librification: Make
`DeviceUnderTest` a `contextmanager`).

LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester