Correct mentionings of refractive index

[jlaehne]

As pointed out in https://github.com/hyperspy/rosettasciio/pull/25#discussion_r975075990, we mixed refractive index and permittivity of air in the documentation. Is now made consistent. Additional reference added.

Progress of the PR

• [x] docstring updated (if appropriate),
• [x] update user guide (if appropriate),
• [x] ready for review.

[codecov-commenter]

Codecov Report

Base: 98.36% // Head: 98.36% // No change to project coverage :thumbsup:

Coverage data is based on head (f43f1ed) compared to base (fce428f). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #147 +/- ## ======================================= Coverage 98.36% 98.36% ======================================= Files 11 11 Lines 550 550 ======================================= Hits 541 541 Misses 9 9 ``` | [Impacted Files](https://codecov.io/gh/LumiSpy/lumispy/pull/147?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy) | Coverage Δ | | |---|---|---| | [lumispy/signals/luminescence\_spectrum.py](https://codecov.io/gh/LumiSpy/lumispy/pull/147/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy#diff-bHVtaXNweS9zaWduYWxzL2x1bWluZXNjZW5jZV9zcGVjdHJ1bS5weQ==) | `94.94% <ø> (ø)` | | | [lumispy/utils/axes.py](https://codecov.io/gh/LumiSpy/lumispy/pull/147/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy#diff-bHVtaXNweS91dGlscy9heGVzLnB5) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy)

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[jlaehne]

@LMSC-NTappy