Add Fold testable example

[taman9333]

I just found that testable example for Fold function was in the wrong place & is not correct

[taman9333]

@samber

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.83%. Comparing base (1c2de46) to head (8ecf736).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #45 +/- ## ======================================= Coverage 88.83% 88.83% ======================================= Files 15 15 Lines 1164 1164 ======================================= Hits 1034 1034 Misses 115 115 Partials 15 15 ``` | [Flag](https://app.codecov.io/gh/samber/mo/pull/45/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/samber/mo/pull/45/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe) | `88.83% <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=Samuel+Berthe#carryforward-flags-in-the-pull-request-comment) to find out more.

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[samber]