DOC: fix bug in custom StacIO example

[martinfleis]

Description:

There is a bug in documentation in an example of a custom StacIO class caused by a mismatch of argument and variable names. This PR fixes it.

PR Checklist:

• Documentation has been updated to reflect changes, if applicable
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Base: 94.31% // Head: 94.31% // No change to project coverage 👍

Coverage data is based on head (0822543) compared to base (a897ff7).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #879   +/-   ##
=======================================
  Coverage   94.31%   94.31%           
=======================================
  Files          80       80           
  Lines       11923    11923           
  Branches     1130     1130           
=======================================
  Hits        11245    11245           
  Misses        496      496           
  Partials      182      182           

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[duckontheweb]

Looks like there are a lot of formatting changes to the CHANGELOG included in this PR. They all look good to me, so I'm fine with including them.

@martinfleis Do you have some kind of linter that you ran on that file to generate those changes?

[martinfleis]

Do you have some kind of linter that you ran on that file to generate those changes?

Ah, sorry about that! Yes, that is gonna be caused by markdownlint VS Code extension.