fixing auto update of default docs by removed trailing / in path

[shimwell]

Description

When we release a version the default docs should get automatically updated but this action
However when releasing version 1.2.0 the default docs didn't get automatically updated.

This is the first time the action has run so it is perhaps not a total surprise

the "Deploy to github page" stage posted this error but didn't fail
cp: no such file or directory: /home/runner/work/GEOUNED/GEOUNED/_build//.*

So I think changing this line
publish_dir: _build/
to
publish_dir: _build
will fix the error

I noticed the dev documentation updating action has already had this change implemented and that action runs much more often

Checklist

• I'm making a PR from a feature branch on my fork into GEOUNED-org/GEOUNED/dev branch
• I have followed PEP8 style guide for Python or run a formatter such as black or ruff format on my code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works