fix: fix mistake in normal_effective_temperature()

[34j]

I think you missed the brackets

https://link.springer.com/article/10.1007/s00484-011-0453-2

[FussyDuck]

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[tlmquintino]

Hi. Many thanks for your PR -- this was definitely a mistake of parenthesis in the wrong location.
I've reimplemented myself instead of merging your change since you hadn't signed our CLA, and Ive corrected the unit tests.
This is now part of the merge in #38 and the 2.1.0 released today.