more docs fixes

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (6d1b66d) 87.38% compared to head (daace70) 87.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #140 +/- ## ======================================= Coverage 87.38% 87.38% ======================================= Files 13 13 Lines 2314 2314 ======================================= Hits 2022 2022 Misses 292 292 ``` | [Flag](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/140/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/140/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | `87.38% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/140?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | Coverage Δ | | |---|---|---| | [src/pem.jl](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/140?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#diff-c3JjL3BlbS5qbA==) | `90.90% <ø> (ø)` | |

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