Remove preserve_underscores as an argument in docstring example usages of datamodel.basemodel.Serializable.write

[terrycojones]

I don't know if there are more instances of this that should also be changed. But passing preserve_underscores to write (as opposed to using when reading) in version 5.0.1 gets me a TypeError:

TypeError: Unrecognized or unsupported arguments: {'preserve_underscores': True}

[mmore500]

Thanks for bringing this to our attention @terrycojones! Will take a look and follow up shortly 👍

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

Project coverage is 88.69%. Comparing base (a065e61) to head (9e2c328).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #204 +/- ## ========================================== - Coverage 88.70% 88.69% -0.01% ========================================== Files 114 114 Lines 12817 12817 ========================================== - Hits 11369 11368 -1 - Misses 1448 1449 +1 ``` | [Flag](https://app.codecov.io/gh/jeetsukumaran/DendroPy/pull/204/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeet+Sukumaran) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/jeetsukumaran/DendroPy/pull/204/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeet+Sukumaran) | `88.69% <ø> (-0.01%)` | :arrow_down: | 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=Jeet+Sukumaran#carryforward-flags-in-the-pull-request-comment) to find out more.

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[mmore500]

Checked the documentation, and you are correct in catching this. Thanks @terrycojones !